Sounds like your drainfield may be getting saturated, and not allowing the effluent water to leach away, or you may have root problems, etc. A cinderblock tank also may be taking in a lot of rain water now that overloads the drainfield.
When is the last time that you had the tank pumped out? Here in NC, the state experts advise us to have a septic tank pumped out at least every five years. Try that first.