Welcome to healthcare Magic!
You have stopped birth control pills three months back and now on unprotected sex means you are susceptible to get pregnant and the severe deviation from your normal pattern of menses-just spotting means you are pregnant unless proven otherwise.
Take a home pregnancy
test on your urine sample and if still in doubt then you will need serum beta hCG
test which is more sensitive for pregnancy.
Please get these investigations and if you like, inform me in a Fresh Query / Follow up Query for further fine tuned opinion.
Wish you a great health!