When to take a HPT after first time sex and pains after sex

Patient: I had sex for the first it a week ago today. After my lower stomach hurt for three days and I had light cramps. My friends have told me it could have been his size and all the hormones from having sex. I am unsure of when my period will come because I am very irregular and have been for years.It should come around the 10th of January but it is a few days late everytime changing the dates for the next. I also take medicine for a under active thyroid. I have read that some thyroid medicines can give false negatives with pregnancy tests.I have been dizzy and urinating a lot but I am also sick with a sinus infection. I am very scared of being pregnant and I am unsure of when to take a HPT. So when would it be best to take a test? What could those pains be after sex? Thank you for the help!