Feeling Lightheaded, Heart Pounding. Took Antibiotics For Infection After Vaginal Delivery. Had Diarrhoea, Subsiding Now. Reason For Faintiness?

Hi, I m a 44 yr old female, who has a 7wk old newborn (also have a 6yr old). I m feeling a quite lightheaded this past week or so, and also seem to really feel my heart pumping when I lay down (have never had this before). My newborn had RSV at 29 days, and I was letting her sleep on my chest alot at night, so I thought that might have something to do with me feeling my heart (maybe the weight of her bruised my chest??). I was taking Palafer during my pregnancy, and just recently stopped taking that (maybe 10 days ago), as I had constipation and didn t think I still needed it (maybe that s affecting my lightheaded/faint feeling), and I also just completed 10 days of antibiotics on Dec 26th ( Cephalexin ) for an infection related to my stitches (I had a vaginal delivery ). To top things off, I developed some kind of stomach bug on Dec 27th, that then turned to very watery diarrhea these last 4 days. Feels like it might possibly be subsiding today, as I haven t had to run to the toilet yet but definitely not back to normal. I m 5 4 , 120lbs, normally very active and in shape but not lately. I take .05mg of Synthroid and a multivitamin daily, otherwise no other medication. Non-smoker.Thank you for your professional opinion.