If your cell medium still looks clear and normal, my first guess would be mycoplasma contamination. You can try to validate by commercial kit, or PCR, or DAPI stain.
These are general signs of cell unhappiness. This may be due to mycoplasma as mentioned above. It may also be due to cell age (what passage are these?).
Decrease in elongation, or the stretched appearance, is often due to decreased adherence to the flask or well. This can happen from age of cells or insufficient nutrients.
Thank you for the support. The cells are in passage number 19. I have even tried with 15% FBS still the problem persists.Please suggest what should i do. Mycoplasma contamination is negative.