Sleep regression?

Is it sleep regression? LO currently 3.5 MO. For the past few weeks she slept at around 930pm after drinking close to 120ml eBM and then slept till next day around 11am ish (wake for milk again at 2am and 6am - quite fixed). However today she fights sleep (no matter how we pat her/coax her), refused milk, and only managed to sleep at 1130pm. Moreover her naps are getting shorter - only slept for 30 mins max, sometimes only 15mins and always wake up crying.