My jade plant has just started to develop these brown/black spots since I moved it inside for winter. Why is it doing this? Am I overwatering? Or too much sunlight now?

Yes, you can cut. Feel the stem. Is it squishy? You will want to cut the stem back until you reach healthy tissue (start at the roots, of course). I see some brown on the stem. I would start cutting there, and work up. Keep cutting until you reach tissue with no black or brown. You want creamy white-green. After you finish cutting, dust with cinnamon to discourage disease and rot. Let the cut stem sit out of soil for a few days (in a dry, dim, cool place). the cut end will feel rough and hard (not moist or juicy). Put it in new DRY soil. I would suggest using 60% perlite and 40% cactus and succulent mix. wash the perlite to get out the dust. Don't breathe in the dust, either- it is harmful to the lungs. Prop up the stem in the soil with rocks, so it stays upright. In a few weeks, you should see roots :) Don't check often for roots, or you may break existing ones. It is best to wait until you see new growth, and then you can water a bit. The soil you have in the picture looks like it has a large contingent of peat. Peat holds water for a long time, and that rots the roots. By doing the above mix, you can avoid rot, but still be careful with watering. To check if you need to water, you can use a wooden skewer. Poke it into the soil, and if it comes out dry, you can water. Good luck! ~palmsandsnow

Sarah I grow most in plastic pots (terracotta are heavier...) But if it is very humid, there could be fungus. For serious fungus, there is a fungicide that could be used (I never used it, so can't speek from experience). I would also consider oedema: it happens when plant takes in more water then it can transpire. This is a physiological disorder, not a disease. I would first repot into more inorganic mix and as I suggested, perhaps reduce number of plants in a pot. I would prune all those lanky, leafless parts off. You could prune plant quite severly, it will regrow. Here is example: old, etiolated plant on left, pruned on right: New growth on pruned plant (I pruned off that branch growing toward center, with many leaves, too): (I could have pruned it even shorter, but will probably split the stems at the base.) Plant above is in full sun from early am untill about 1pm now. It grows healthier and much more compact.

Being 'near south facing window' is nearly not enough light. It may be not enough even if plant is on the windowsill of S facing window - depending if the window is obstructed by trees or shrubs or other buildings, and your general location. Jades are one of the easiest plats to grow - I am saying that from personal experience. BUT, they should be potted in well draining potting mix, in a pot with drainage hole(s), and watered when the mix is dried up. If you do that together with making sure plant gets good light, jade will thrive. Period between watering depends on temps plant is in, and certainly on a mix you use. If it retains water, that creates problems. I already suggested how to improve the mix. I also see that your plant is double-potted. It could be - lots of ppl use cachepots. But it needs to be taken out of decorative pot to water, let the excess water drain, and only then put back. Plants in the photo were outdoors, in sun most of the day, since spring to fall. Photo is taken in beg. of December just after brought indoors. They are right on the windowsill of unobstructed south-west window, and they will also get light from LED lights.