An ectopic pregnancy can, from what I have heard from women I know who have experienced one, be very painful. It can cause vaginal bleeding similar to either a period or another type of miscarriage. Especially if it goes untreated and it ruptures a fallopian tube, this can cause terrible pain. I suggested this as a possibility to your uterine discomfort given I assume you aren't 100 percent certain it is your uterus but rather somewhere in the vicinity of your female reproductive organs.
If this continues or is getting worse, please seek medical attention. I cannot begin to tell you what it is but it would be prudent to find out what it is before it potentially becomes something bad.
Here is a link to a very reputable website for the Mayo Clinic. It gives good information on ectopic pregnancies.
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