Sounds like you might have cut a tendon! How deep was the cut? Did it require stitches? Control of all our fingers, whether bending or straightening, comes from muscles in our forearm via tendons thru the wrist. So it sounds like you might have cut one or two. Or had it stitched accidently while closing the cut. Go see your family physician. S/he might have to send you to a hand specialist to get it fixed. Occupational therapy afterwards will help.
BTW, did you get a tetanus shot after you got your cut fixed? Don't forget! And in case you have close contact w/infants (like your grandkids), you'll want to get the tetanus shot w/pertussis included (TDaP), to prevent from spreading pertussis to infants who haven't had a chance to be properly immunized yet.
I know, it's just a cut and your index & thumb aren't bending. But I'm a family physician and we're trained to care for the whole family. Good luck!
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