Why am I getting stretch marks?

Im ten weeks pregnant, and I am getting stretch marks everywhere! since I became pregnant, I have lost roughly ten Kgs, I have put another 4 or 5 kgs back on but have been maintaining my weight, but I am getting stretch marks everywhere! on my arms, backs of legs, all over my tummy! but no weight gain! I think I may have a thyroid problem, what do you think? I don't eat very much, I have a small dinner and a few peices of toast over the whole day