Cells become round and detached after changing to fresh medium in plate

I have been trying to transfect an endometrial epithelial cell line with a plasmid. But before transfection, when I change the medium of the cells and replenish with the same growth medium in which the cells have been seeded, the cells turn round after addition of fresh medium. I see round structures that are greyish black in colour. The cells start losing their shape, become round and start to detach and are afloat. The structures that I see look like lipid droplets and some such structures have a shrunk debris kind of thing attached to it. The media I use is DMEM+Hams F12 and supplement with penstrep and heat inactivated FBS.