15 weeks baby nursing strike. Anymore.

My 15 weeks baby is on a nursing strike from past 5 days. Since her birth I have been using nipple shield to breastfeed as I have flat nipples. At her 10 weeks checkup the paediatrician asked us to give formula as a supplement for one/two feeds as her weight gain was not appropriate. She would spit the formula and start crying, so we would kinda forcefeed her. Was feeding formula sometimes in a bottle and sometimes using spoon. Either way she would cry and make a fuss. Now she is refusing to breast feed. But at night she drinks in that semi awake state. Only when she is awake that she is refusing to breast feed. Any suggestions to get her back to breastfeeding. I'm still giving her formula for 1 or 2 feeds but not using bottle anymore.