thanks for sharing your concern.
it might be because of chronic infection in your tooth.a well treated root canal is free from pain and infection,post treatment.
An ENDODONTIST can solve your problem by reentering into the canal and treating them.
you have not mentioned which tooth are you talking about?.Because in posterior(back) teeth ,there may be 3-4 canals are present in tooth.
In proper root canal treatment
all canals should be traced and free from infection ,then only lump will go.
Now a days latest root canal system WAVE ONE facility is available . root canal treatment can be performed in 1hour only with that.
This procedure can be done without medicine because you are pregnant . look for an ENDODONTIST for this latest treatment ,he will solve your problem .
hope it helps.