I was taking minocycline for acne but had to stop taking it because of the nausea and dizziness. i also take tri-lo-spintec. i do not miss a pill ever. i had sex 5 days after going off the minocycline. should i worry about effectiveness of bc?

Possibly. How long did you take lt? Although rifampin is the only antibiotic that's considered to render oral contraceptives unreliable for birth control, a few cases of other antibiotics including tetracyclines, to which class minocycline belongs, interfered with oral contraceptives. If you were to go back on rather longterm minocycline for acne, another form of contraception may be advisable.