Worried I'm pregnant (again)

I just had my twins in November, so they're almost 4 months old now. I've been on birth control for three months now and using condoms as well. There was one incident recently of unprotected contact but no unprotected sex. I should have started my period this week, but have yet to do so. My worry spiked though when I took a test that originally came out negative but I later looked and found a faint second line (although this was well after the appropriate time frame for the test). I know breastfeeding women don't always bleed during a period and that there's a good chance what I saw was an evaporation line, but I really just need reassurance of that.