This can be due to injury of the extensor ligament of your finger called as vicket keeper's finger, generally a brace or spica is to be used to keep it straight as it gets slightly bent along with swelling and pain. But now you should consult a physiotherapist.
Next Steps
Go to a physiotherapist.. Ultrasonic therapy +wax therapy +some strengthening exercises will be helpful.
Health Tips
At home.. Rest +hot water fomentation +pain killer gel +frequent finger movements +avoid cold exposure.