Back Pain during Core exercises

[Corona]

Had to stop doing my core workout (lots of sit ups, oblique sit ups, flutter kicks and bicycle crunches) for about 2 months due to just ripping all the skin off my tailbone. I've gone to start again today with my old routine but reduced reps, but straight away I'm getting bad lower back pain during flutter kicks, which I never used to get full stop. My form is fine, no arching of the back and my core while not as strong is still capable of the old 85 sit ups to the beep with good form.

Does anyone have any idea what could be causing the pain/how I can stop it? Any help is appreciated greatly

Thanks