could be lemon balm, which is a relative and looks similar. More than this, I'd need a picture.

There is also a Lemon Catmint that looks like regular Catnip. If your plant has a "skunky" smell(don't know how else to describe it, though not as bad as real skunk) in addition to the lemon, it is probably Lemon Catmint. If it just has a nice lemony scent, it is probably Lemon Balm. Just the skunky smell(which goes away when dried) would be the Catnip and your cat would be all over it. I've grown all three and done numerous cat tests :D All three have similar leaves, but I believe the Catnip leaves are larger than the other two although this may depend on growing conditions. Hope this helps.

Just to add to the confusion. Catmint (Nepeta faassenii) is often also commonly called Catnip, and while some cats go for it, a lot don't. Its smell is much more pleasant than that of Catnip, which is Nepeta cataria. If you really want your cat to go ape over a plant, get Catgrass (Dactylis glomerata).

Catnip and Catmint are two plants which are commonly incorrectly labelled.

Here is a picture of catmint(Nepeta mussinii), it is in the right hand side of the picture.

{{!gwi}}

Mine blooms regularly from spring to fall and stays in a neat small cushion shape. I also have six hills giant catmint (Nepeta faassenii) which does resemble catnip more but has no strong scent.
Catmint (Nepeta mussinii) has a nice mint scent and is good in dried arrangements and potpourri, but do NOT put it in a arrangement in water or it will make the water smell like sewer. I don't know why it has that bad habit when it smells so great dry!

Catnip has a "fug" smell to it, and is a tall gangly plant.

I have catnip (Nepeta cataria) in my yard to use as a mosquito repellent. It works. I live in a rural area and in the past, whenever the farmer planted corn in a field west of my property, the mosquitoes would attack me in great numbers. I had to cover myself with Deet. If I missed a spot applying the Deet, the mosquitoes would go after my ears for example. It got so bad that I bought one of those propane foggers to clear out my backyard of mosquitoes. The mosquitoes must love corn fields. Whenever the farmer planted soy beans, the mosquito population wasn't so bad.

I read about catnip (Nepeta cataria) being a great natural mosquito repellent so I bought a pound of seeds and planted catnip seeds on the west part of my yard, next to the farmer's field. After about 2 years, the catnip spread like crazy. It became invasive in a way. I believe that birds eat the seeds because I had catnip growing in my front garden (east). I pull out catnip where I don't want it. My catnip grows over 5 feet tall and I use those 5 foot tall tomato cages to keep it upright.
Another thing about my catnip, it has a mild scent of a skunk when I walk through it or when a breeze blows through it.