What Causes Light, Frequent Periods?

Period twice in one month. Last period was regular for me. This one is much lighter, but still heavy enough to soak through underwear in the hour or so I was at church, before I noticed. I have been on the min-pill for birth control for a couple of years. I am 23, female, 5'7" and 140lb. I had trouble with irregular periods when I was in my very early teens, not since then. I have lupus and take levothyroxine, hydroxyquchloroquine, and lithium carbonate as well as vitmin d and fish oil.