Why am i getting such horrible stretch marks during my pregnancy?!?

Im 39 weeks pregnant and it seems like everytime i look in the mirror i have another stretch mark! I have them in my thighs like you wouldnt belive, their hideous & i dont know what im going to do. There horrible on my stomache too. I just want to know why im getting them so bad.. i use lotion. I dont eat unhealthy, i rarley ever eat any junky food. I dont drink soda. I just keep putting on weight and getting these marks, any idea why im getting them so bad??