Woke up w/ about 20 (mosquito) bites in mexico, varying sizes up to a few 2" welts. allergic reaction? what 2 do?

Yes, reaction: 2 inch welts x 20 can be very uncomfortable. Cool packs and benadry (diphendydramine) can help. I've also seen baking soda help. Watch for signs of infection.

Antihistamine: You can apply cool compresses to the bites. If you have any antihistamines, now would be a good time to take one--like Allegra or zyrtec, or if you are going to sleep, benadryl (diphenhydramine). If the mosquitoes are still around, you may still need some insect repellent! depends on what you can get hold of at this late hour...You can apply cortisone or antihistamine cream to the bites, or rhuligel or sarna, eg.