there seems to be a slightly higher rate of vaccine
administration, Indian academy recommends 5 total doses as per schedule and all the pulse polio
programme days that is conducted twice a year.. this is basically because of poor uptake of polio virus from the vaccine in our subcontinent.. but every two months that you mentioned sounds like quite a high number of doses.. also in very rare cases the polio virus which is attenuated(weakened just to evoke protective response not to cause the disease per se) may end up causing mild polio like illness characterized by weakness /pain in the lower limbs
with mild fever
which recovers gradually with supportive therapy only..
i would suggest you to visit a near by pediatrician
and discuss regarding this problem as examination of the child will provide vital information and also may get the stool sample analysed for the type of polio virus if it is similar to one in vaccine or if there is any mutation
also at this age there is a good possibility that the child is growing due to excess activity.. the lengthening of the limb bones itself can cause mild pain known as growth pain which resolves by itself..