What could this backache cramping be from?

So my fertile window was from September 6-12 . The 10th was the highest fertile day so I should have ovulated that day. I had cramping the 10th on one side of my lower abdomen like it could have been expected ovulation and I had some a little bit the day after on the 11th too. Yesterday the 15th i got a terrible backache that was similar to my back cramps I get either 1 or 2 days before my period starts, only problem is that my period is expected to start the 25th so 10 days later. When I got the backache/cramp it was followed by a hot flash and then nausea. The back pain went away after a few minutes of having my heated seat on but the nausea didn't go away until I woke up this morning. I had unprotected intercourse on the 6th which was the first day of my fertile window and on the 13th the day after my fertile window. I doubt this could be related but just thought I'd put that information there just in case.