Hormonal Shift?

Hi! I've been struggling a lot lately with my emotions and I'm wondering if anyone else has had a similar experience...

I've always had pretty severe mood swings,irritability, depression and I was on medication for a long time. I was severely depressed when I was pregnant but after I had my baby I did a complete 180. I had occasional irritability (probably due to lack of sleep) but that's it and I had completely stopped taking my medication. But once my period returned I went back to my old emotional state of being constantly irritated, moody and depressed. It's like a switch was flipped on then off then in again. The only thing I can think is maybe a hormonal thing? Has anyone else felt with this?