What could cause unusal/abnormal menstal cycle changes?

I am really good with making sure my personal hygiene is close to God like practice-- I knew I was pregnant 3 days after conception, good....

My last period was not like my other periods. I had a red-tinted discharge that last for a week but not enough for a tampon or pad to be needed, only for 3 days of heavy bleeding to occur, than gone!!! Stopped.

Usually my periods are on time and regular, super heavy flows. I am too young for menopause. I am kind of concerned here. I am sexually active with my husband, but I am not pregnant (already took several tests, even from to OB); I started taking a new OTC multivitamin about 2 months ago. Nothing has changed in my diet or activities.....

Can someone explain to me what is going on?