I've been a victim of this before. This is bad news.
In my (rare) case, the roof of my apartment building drained to the middle of the building and down the sewer pipes. During phenomenal downpours (twice in five years), the flow of water coming down that pipe was faster than the flow of water leaving the building. The water backed up to the lowest point where it had an exit which was my primary bathroom. It did so with such intensity that water was violently sloshing out of the toilet bowl, the stopper on the sink was forcefully ejected, and the bathtub was backing up at the same time. The only good news in all this mess is that it was fresh water.
Clearly, this is not quite your case. However, it is an indication of the same similar fact: you have a flow going in the wrong direction in your sewer line. Before the astonishingly violent (I'll post a photo of the sink shooting water over a foot high later on) discharge and water flowing everywhere, the toilet would gurgle.
Since it's a septic tank for you, you need that serviced now.
The reason the flushed water level is lower is probably that the siphon link between the bowl and the sewer line isn't being broken.