Hand Spasms during fingering / oral ??

When my boyfriend is fingering me or giving me oral my hands go into a 'crab claw' shape and I can't move my fingers.

My legs also shake a little bit.

When this happens I don't know whether I feel pleasure or pain so I either tell him to stop, or, sometime I feel like it might be an orgasm so he carries on but I don't.. orgasm...

Could someone explain this to me please?

or tell me how to stop my hands having a spasm?

Or what else my boyfriend could do to make me orgasm instead of the standered fingering?

Thanks :)