It is possible that because of your age and your recent miscarriage, your periods are simply going to be irregular for a while. However, I would strongly suggest that you call your doctor and ask.
Also, I'm not sure what you mean by "dry sex." However, here at Scarleteen, we recommend that you always use lube with your condoms. In addition to feeling nice, it helps keep the condom from tearing. See:
It is unlikely that sex with your boyfriend caused anything to tear, especially since you've had sex before. However, some women find that having sex during or right on the edge of their period stimulates blood flow.
Again, I would really recommend you call your doctor. Most likely nothing is wrong, but we can't diagnose you over the computer, and it's better to be safe than to wonder.