Hey guys, thanks in advance for any help.

The situation:

Two nights ago, the bathtub in the second story bathroom was used and fully drained all at once. I happened to be near the main floor bathroom and I heard the toilet gurgling/bubbling at some point during the tub draining. I could see large air bubbles coming out into the bowl. I tried flushing this toilet and the water wouldn't go down (raised up to near the top of the bowl). I grabbed a plunger and tried to plunge on it a few times and it did not go down. In the past, when this toilet flooded it would be extremely easy to plunge. I left the bathroom and came back about 5 min later.. the toilet was completely empty of water. Fast forward to today... as I took a shower (again, upstairs bathroom but not the same upstairs bathroom as the bathtub) I could hear the toilet gurgling/bubbling.

So far, I haven't seen any bad repercussions (water on the floor, ceiling, carpet, etc) but I am getting nervous something may be on the way soon... which is why I need your help. What could be the problem here? To me it seems like it is either a) an issue with the main sewer line or b) an issue with the venting in the house (possibly plugged?). I certainly could be wrong there.. and am hoping to get some suggestions on how I can diagnose between the two possibilities or potentially other possibilities that I hadn't thought of.

Thanks!