What Causes Severe Headache And Dizzy Spells?

Hi there my name is Jennifer. 33 years of age with a 21month old baby delivered through section. i was wondering if you could help me piece a few things together, after having my daughter it came to light i had an underactive thyroid borderline which seemingly cleared up after a year. About 3 months ago i started getting severe headahes and taking dizzy spells when i went to my nurse she took my pulse which was 175 i was sent to the hospital and the done a brain ct scan showing nothing the headahes were daily for about 5 weeks and then left leaving mdd with a racing heart and dizzy spells. Im awaiting a 24hour heart monitor but they have said my heart rythem is good. I have a pain in my right side well more of a discomfort which my doctor is sending me for a pelvic ultrasound and done blood tests which all came back normal except my red blood cells are enlarged? I am normally very healthy and even manage to miss the common cold most years. Just wondering if you could shed any light please? Thankyou in advance for your time.