My appetite has been very up and down too, in the first 13 weeks of my pregnancy i lost a few kg due to no appetite, then it came back with a vengance. Bub will take what he/she needs first and your body will soon tell you if it doesnt have enough left to run on. If your worried about bub try to take a daily multivit with iron and folate in it, and drink milk when you can, but dont stress, bub will get what it needs from your body.

Good luck for a happy and healthy pgcy!