Sensitivity is a big problems if you are facing this problem try to use a sensitive paste for a week and if sensitivity persists then you must visit a dentist. Sensitivity can be due to many reasons like:
a) cervical abrasion which happens due to incorrect brushing technique
b) fractured filling
c) chipped tooth
d) dental caries/cavity
e) using a very hard brush
f) gum recession
g) capping a vital tooth
h) improperly done RCT etc
Try to visit a dentist as soon as possible to rule out the etiology behind sensitivity.