mynamesixonethree – Some of the posters have mentioned some of the possibilities. Unfortunately for you -there are many more.
Diagnosing this common complaint requires a workup (sometimes even extensive). So my advice to you is -hopefully you have a good relationship with your PCP and make a long appt. with him/her to get to the bottom of this. You might not get an answer right away, but be persistant. If you don’t have a good relationship with your PCP, find another that you can have this type of relationship. Because how the PCP is able to illict a H & P -is how he/she will decide which direction to go in, eg. tests, etc.