Gosh, it could be anything.
Might be serious or not.
Might be something easily fixable or not.
Keep a diary of symptoms - when you experience them and for how long, what you were eating/drinking at the time, did you sleep well, did you exercise, what the weather was doing (barometric pressure, temp, precipitation, etc), did you change any makeup, soap, lotions, deoderant, laundry products, any supplements, vitamins, etc you take - in short - everything.
Take it with you to your Dr visits.
When they draw blood and take tests, note it in the diary, and what the results were when they came back and what the results/numbers were (and their normal ranges they are looking for) and the medication(s) and dosages, if any, that were prescribed.
Not everyone will agree with this but when I'M feeling out of control, it helps me to collect the data and lay it all out in front of me (and the doctors).
I hope they find out what it is quickly and that it's easily fixable.