is she behind??

My LO turned 4 months on feb 8th. She was born 3 weeks early. She totally hates tummy time and she will pitch a fit until she rolls over or we roll her over. I'm almost sure it's because of her silent reflux. she's always spitting up. We can barely move her and she's constantly spitting up. She does not show interest in toys but sounds doncatch her attention but she babbles like crazy! I've see some of y'all  babies already sitting up, and I know every child is different.. when I put her in the corner of the couch she will sit for a bit but then she will just lean to the side. I practice with her all the time. She picks her head and shoulders up like she wants to sit up and I have her hold my fingers and she pulls herself all the way up to the standing position and will stay like that for a while. 
Should I be worried about anything?
Any tips or tricks?