This morning I awoke to a problem in my house. Flushed the downstairs toilet and the water went half way out then went nowhere. I could hear bubbling in the sink, then the water in the toilet slowly emptied out and air bubbles were coming up through the toilet drain. I tried flushing the upstairs toilet and it flushed fine but the downstairs toilet filled with the water from upstairs. I plunged it (not really expecting anything) and I could hear bubbling in the sink as I was doing that. Also, I could see a little bit of water coming from under the toilet as I plunged.

So...I'm on a septic tank....turns out it hasn't been pumped in almost 4 years. Is it full or is it something else I'm missing?

Well, no need to answer. The tank needed to be pumped out and that solved it.