Okay, I know a little bit about a lot, and have been in many situations. (None with STDs or STIs, but know a lot about them, too.)
1.) Is your new birth control the depo provera shot? That CAUSES irregularity in bleeding.
2.) Even if it's not depo, yes, your body COULD be reacting to the new introduction of hormones.
3.) If you just are recovering from a yeast infection, YOU DO NOT WANT TO USE A TAMPON!!!! YOU CAN CAUSE THE YEAST TO STAY AND MULTIPLY EVEN IF IT'S ALMOST GONE!!!!!
4.) Do not have unprotected sex with your partner, until the yeast infection has CLEARED because men can also get yeast infections from women: THIS IS CALLED JOCK ITCH!!!
5.) You said you no longer have any pain in or around your vagina: BUT HOW BAD WAS YOUR YEAST INFECTION?!?!? There are different kinds, there are different ways they can effect you, and if your infection was severe, it may have even caused problems internally. *Google "yeast infections" and click on "images." LOOK AT THE PICTURES!!! AND THAT CAN HAPPEN INSIDE OF YOU TOO!!!!! I even thought my partner gave me herpes once!!! I was so relieved it was just a BAD yeast infection! If there was a bad issue, such as an internal blister in your vagina that they or you have missed, that may have popped and caused your bleeding.
6.) Spotting can happen for ALOT of reasons! Don't automatically assume various things. See your ob/gyn and be tested for the whole list, if you have coverage. (blood work is expensive.) You may not have been recently tested for HIV infection, or other things that REQUIRE bloodwork. Also, it is important that you know:
RECURRING YEAST INFECTIONS CAN BE A SIGN OR SYMPTOM OF HIV INFECTION. This isn't always the case, but it's important to know!
7.) I recently thought I was having a miscarriage or an ectopic pregnany(where the fertilized embryo attaches itself to the wall of the fillopian tube.) I was in alot of abdominal pain, was spotting, even was late for my period. The on and off of bleeding and brown, even a clear fluid, with a hint of pink coloration, fluctuated, to not even anything at all for about 5-7 days. (Much like you describe.) I finally got worried, and went to the ER. To make a long story short, they did an internal (vaginal) ultrasound, found I was NOT pregnant, not miscarrying, and not having an ectopic pregnancy:
I HAD ONE OVARIAN CYST. This was causing my problems! This still may not be your problem, but these are all things worth looking into. (And ovarian cysts are nothing more than eggs that did not become fertilized, and for some reason, were not shed, but instead attached themselves to the ovary and retained alot of fluid. They are supposed to eventually burst or rupture and go away. It is painful. Many women who get them even get several at a time.) These are also prevalent to women with PID (Pelvic Inflammatory Disease/Disorder.)
Get checked out girl!