From IUD to the pill

An

So I had my mirena IUD for a little over a year. I got it removed may 1st due to developing fibroids and other problems with it. Instant pain relief when it was removed. On that same day of removal I started the pill. Since then i have been bleeding on and off from light to heavy to brownish. Then it will stop and start up again. I contacted my dr and was told it wasn't normal since I started the pill right away and only should have expected a heavy flow during the 1st week since I removed the IUD. But the continuous bleeding is not normal. She then has now switched me to a new pill with a higher dosage in hopes that it will work for me. Has anyone else had this issue with bleeding? Specially when switching forms of birth control..?