Sounds like you need to go get your thyroid levels checked. I gained weight just like you have and it took over a year before I finally got them to check my thyroid and by then I was so hypOthyroid, I was nearly a basket case and had gained a total of 30 pounds. Hypothyroidism will also cause the fatigue, erratic periods, etc.
Do a quick search for "hypothyroidism" and check out the other symptoms.
I have Hashimoto's Thyroiditis (which is autoimmune - antibodies attacking my thyroid), plus I have hypothyroidism. Get checked ASAP. Make sure they test at the minimum: TSH, Free T4 and Free T3. Depending on those results, you may or may not require further tests. Oh, as long as you are at it, have them check your vitamin B12 levels to make sure you don't have pernicious anemia. I have that also and it requires regular injections of B12, but well worth it.
Good luck and let us know how you turn out.
If you're constantly having your period, your iron can be low. That could account for some of the fatigue. I'm sure a doctor prescribed the antibiotics. Have you called their office for some advice?
If you're thinking about the "old standby" then I wouldn't hesitate - get some professional help.