If you have been having sex it is possible that you are pregnant. But it also sounds to me like your mind might be playing tricks on you.
Stretch marks and shortness of breath usually dont occur until you are well into your pregnancy. And the stretch marks on your thighs could just mean that your thighs are growing. Most of my stretch marks occured on my stomach.
Sore breasts and tiredness could be a result of the extra hormones released in your body when you are getting ready to start your period.
If you miss a period, take an HPT. It can never hurt, and it can put your mind at ease. But always remember the consequences of having sex at a young age, because most 16 year olds are not prepared for a baby. If you dont want to be a parent, the best thing to do is abstain. If you continue to have sex, I would suggest getting on some kind of birth control.
