Toilets still bubbling after recent clean out/new filter

We just had our tank emptied two weeks ago, and a new filter was put in. They said there was a clog at the time and they cleared that out. Today our downstairs toilets are bubbling again and the toilet nearest to septic is not draining. Water does not appear to be backing up. Is this a plumbing issue or do I maybe need to replace the pump motor in the septic tank? How can I tell who I should call?