What Causes Lower Back Pain After Recovery From Stomach Infection During Pregnancy?

Hi there, I m 28 weeks pregnant, and just fought off a nasty stomach bug. The following day I feel dull pain (that gets worse) in the lower left back side, it seems to get worse after I eat, and it starts to feel like bad menstrual cramps. Once I lie down and put a heat pad on it gets better. What could cause this? I also have a dermoid cyst on my left ovary that got bigger in pregnancy.