Did anyone have severe side/back pain during implantation?

Well I think I think I got my period a few days ago -- several days earlier than I expected -- so I doubt I'm pregnant, but the only thing that gives me pause is that I had a really severe, abnormal cramping pain in my lower left side/back, which I've NEVER had before. I started bleeding that morning, and a few hours later my left side was so sore whenever I moved or walked that it almost caused tears and I had to leave work (I try never to miss work for menstrual pain), but I soaked in a bath for about an hour and was fine afterward. I continued what looked like standard bleed (maybe a little light, but too heavy for implantation supposedly) the second day, and by the second night I didn't even need protection to sleep... day 3 (today) I'm only spotting a little. Usually AF is very heavy with varied symptoms. Anyone else bleed heavily or experience side pain during implantation?