My periods are 10 days late, I have done 3 tests and all have been negative. My periods come the same time each month, my cycle is very reliable, I don't know what to do next, could I be pregnant?

Could be an off cycle. You can continue to try a test at home every several days if it doesn't show (would get a bit pricey), see about having a test done at the doctor's office, or wait until your next period would be due and see if it too doesn't show and test at that time.