Plumbing and septic: bubbling and backing up
Simple answer: yes, it could be any and all of those. In my experience, clogged main pipes is the most likely cause. (See below for more detail.) If you cannot use a motorized snake yourself to clean out the system, hire a professional. There is also a chance the septic system needs to be serviced. 

Recommendation

Hire a professional plumber who can use a motorized drain snake ASAP. We had a similar issue, neglected it for a year, and during a visit from the in-law, the toilet decided it was done and overflowed. The HVAC system was also a victim (in-floor registers). By waiting until two days after the overflow, we saved a few hundred dollars over an emergency call, but we could have saved more by addressing it quickly. 

The individual who does the "snaking" should be able to snake the entire system, from all clean-out locations (our system had multiple). Also, if the system has vents which can be compromised (squirrels or other pests with nuts etc.), consider having those cleaned if the price isn't prohibitive. The company who provided the service to us charged by the hour a much reduced rate after the first hour had elapsed. If we had the option, I would have preferred this. (In our case, the service personnel had two more calls to make after finishing our property after 5 pm. I was told that if anything wasn't completely working properly, the servicer would return on their dime, and I wasn't about to keep someone from their family around the holidays.)

Update

I can now say from definitive personal experience: hire a quality professional to minimally snake and clean the system - but preferably have them do an inspection as well. A 

Do it right, right away.