The over-the-counter tests are pretty accurate, but may not show a positive result until more time has passed than your doctor's blood test will reveal. Since you said that you are peeing a lot, it is possible that your urine was too diluted for the test to detect the HCG.
Also, a tender uterus is NEVER normal. Sounds like you might have PID or some other issue going on.
As for your baby not liking your milk, that could be from a lot of things, including any medication you are taking, a new food you have tried, or any of a long list of things.
Since you have already been pregnant three times, you know what your body will do. Think about the things that happened those other times: are your breasts sore (may be hard to discern since you are still nursing), do you crave certain foods, do some smells make you nauseous, are you moody and cry easily? Are your husband and other kids on your last nerve?
In any case, I would go to the doctor about the uterine tenderness and have him draw blood for a pregnancy test.
Good luck!