Can the teeth under bonding get cavities or decay?

You can get decay under bonding.  I would see a dentist soon to have an evaluation and determine if there is decay present.  If so, take the dentist's recommendation and have it treated.  Do not wait until you have bad pain to go or you could make the potential problem worse. Good luck. 

yes, that is possible. dentist can check that by xray. but sometimes its not possible to check by xray in such cases removal of existing filling is must.

Bonding is a composite(plastic material) which is tooth colored.Bonding is usually is placed do to decay in the tooth or as a result of a fractured tooth.If your tooth is aching, there is a good possibility that the bonding has decay under it.The extent of the decay determines what needs to be done with the tooth.If the decay is shallow, it is possible to re-bond.If the decay is deep, there is a possibility that you may need a root canal.I would see a dentist to check the extent of decay, as soon as possible.

It is possible that decay can get under an existing bonding.but it is also possible a nerve could be going bad .i suggest you have it looked at by a dentist