Ovulation question...

Has anyone ever ovulated at the same time ever month and then one time it been earlier? I've taken ovulation test every month since my surgery. It was always positive on my peak day. I had the cramps then and everything. This month I decided that I'm tired of spending the money so I decided not to use opks. Well this month I got my cramps on the day before my fertile week started. Since I didn't take test I don't know for sure when I ovulated but I haven't had cramps since. I just feel like my down stairs is sore, almost like I've been working out but I haven't. I've also have a slight headache everyday. Not bad enough for meds