If it is indeed just due to tightening up the house and not some other reason, then you might need make-up air. The need for makeup air will be more pronounced when using higher volume air movement devices, such as a kitchen fan.
Depressurizing a house can cause problems with reverse flow of exhaust gases down flues, for example.

A good way to bring in new air is with an HRV (heat recovery ventilator).

Prior to just popping in an HRV, realize that by tightening up a previously drafty house you have reduced air exchanges through the house. With fewer air exchanges, moisture generated within the house (bathing, cooking, etc) isn't being removed like it once was, so it's likely that the relative humidity within the house has increased. So any minor increase in moisture levels from showering could be enough to trigger condensation on windows, etc.

So yes, it could be that your fan is moving less air. Or it could be that the base moisture levels within the house are now higher. Or it could be both.

It's a good discussion to have with whomever will do the work, air exchanges versus dehumidification...One of the two might be needed, or it could be both.

A door blower test could tell you just how tight your house is and how many passive air exchanges per hour your currently getting.

Mongo

Your house is not breathing like it used to. While you probably didn't notice it, your old house was always drying out from the moisture by cooking itself from the inside. Works, but not very efficient.

Thanks for the replies!

I have noticed that the humidity in general is higher in the house this winter now that the Tyvek has been installed.

If a HRV expels the same volume of air that it draws in, it doesn't seem like one would help the bathroom exhaust fan issue.

What should I expect to pay for a "door blower test"?

Another thing. When I had my high efficiency propane furnace put in several years ago, I was told that it didn't need to have anything to bring outside air in for the combustion since I had an "old" house that wasn't "tight". Do I need to do anything for it now?

These are "nutshell" answers since envelope issues are tough to diagnose over the internet.

The HRV is an exchanger, but if your house is tight and your winter interior humidity is high, it'll exchange stale humid interior air for fresh dry exterior air while minimizing heat loss during the exchange.

A blower door test is usually part of a more comprehensive analysis which could run a few hundred dollars. $200-$400 depending on the thoroughness of the inspection.

If your house is excessively tight then it might make sense to install make-up air duct for the furnace.

If you have an HVAC company out there, often times they'll do envelope and blower door tests. With all the "green" garbage going on, there are a lot of folks jumping on the bandwagon just as "energy consultants." If you're going to have work done, many companies will credit the cost of the blower door test and other analysis towards the cost of future modifications.

Backtracking and peeking at the larger picture, basements can be a source of moisture vapor. Moisture vapor and maybe even liquid through rubble foundations, or vapor through poured concrete or block foundations.

Think of it this way: You tightened up your house, which lessens moisture removal, but if your basement is a source of moisture vapor, it's still adding moisture vapor to the house. If it applies to you, a good thing to do is to minimize as best as you can air movement from the unconditioned basement into the conditioned living space above.

Go in the basement and look up. Any gaps in the basement ceiling that lead into the house above (plumbing penetrations, electrical penetrations, hvac penetrations, etc) can be sealed with canned foam.

It makes sense to do the same thing in the attic floor if the attic is unconditioned.

The goal is to keep the conditioned air that you're paying to condition within the thermal envelope of the house.

It's not a sexy job, mostly elbow grease, but walking and crawling around your house and shooting gaps with foam can have great effect. With those DIY cans of foam like Great Stuff, it's best to uncover the gaps first, then fill them all at once. Ling delays between trigger actuations can allow foam to cure in the nozzle.

From my experience the make-up air come from under the bathroom door and there's more than enough in the house. If you have to temporarily open the front door to test further, by all means.

I would start with the more obvious stuff. Did you check your flap on the outer wall so it's opening freely? I had a client with this problem, and the reason was that the outer flap was obstructed by a screw used to install some hardware cloth to keep birds out.

Or is the siding obstructing it some other way? You could reach in or peer in with a flashlight and handheld mirror. Did the duct get kinked or disconnected somewhere else? If the motor runs the same as before, it should work the same. With the cover removed, test with your finger that the little exhaust flap in the housing is moving freely.

I apologize for not being able to quote a source but I KNOW I read somewhere about someone having to cut an inch off the bottom of their bathroom door after having their house wrapped.

There just wasn't enough air being drawn in to get a flow going.