I'm an 18 year old girl with severe hashimotos. i'm on birth control but have had unprotected sex. my last period was very light; could i be pregnant?

Unlikely: If you have adhered to your BC program and not missed any pills, the failure rate is cited as less than 1%. If concerned do a urine pregnancy test on a first morning specimen weekly for a while to be sure. Light periods are common on some BC.

Probably not, but...: A urine pregnancy test, done correctly, gives a reliable result 3 weeks after sex (3 weeks after when conception might have occurred). If done sooner, a positive result is likely real, but a negative result may be due to testing too soon, and thus lead to a repeat test. If a woman gets her normal regular period month after month, then she is not pregnant, and a test is not needed.