Hello Jacquie
You are right to be careful.
Good news! That feeling is normal. If you are holding on and pulling hard on your heels, squeezing your hips moving forward, lifting your chest etc then this is an intense backbend that can make doing the sit-up that follows a challenge for some. This happens to me when I hold Camel for a long time (upwards of a minute).
Then main considerations follow:
1) Make sure that you do stretch up before going back (just in case you are ‘crunching’ into your lower back.
2) When you release from the pose (as slowly as you like or need to) make sure that you do not flop forward into any sort of forward bend. Come up to vertical, head up last. Rest back down onto your heels and try to lie down carefully without undoing all your wonderful hard work.
3) Lie on your back as still as you can. Make the assessment whether or not you think you should do the Sit-Up. If the answer is no then bend up the legs, roll to the side without twisting your spine, place your palms on the floor under your shoulders and push up from the floor while looking at your hands (so as not to strain your spine at all).
I did notice you said you hold onto your ankles. Was that right? Or do you hold onto heels?
Namaste
Gabrielle