Very light period and a late

My periods are usually normal. My cycle is about 28-30 days and my period lasts 7 days. I normally have a medium to heavy flow with clots and I have to change my pad 2-3 times a day. This month, my period was 10 days late. I took 4 pregnancy tests and they were all negative. I also took a blood test and it was negative. A week before my period was expected, my breasts were very tender, which is normal for me.

Now I have my period, I think. It's very light. I'm only having to change my pad once a day and it's not even full. It's pink/red when I wipe, not nothing like my normal period. Any suggestions why my period is acting up?

I am sexually active with my boyfriend, but like I said the tests are negative. I'm on blood pressure medicine (lisinopril), but I have been for a few months and this is the only month I've been irregular.

Thanks for reading!