Our upstairs tub draining causes the toilet on the bottom floor to overflow with warm tub water from above (not sewer)... water only. I don't know the routing/piping on a two story house with regard to plumbing but any ideas? Washing machine running at same time on cold.. no other problems were seen, but when we stopped draining the upstairs tub, the overflow stopped on bottom toilet. Where to start? Without knowing routing, would I pull and snake the bottom toilet discharge? Is this a venting issue? Any help would be greatly appreciated!

Thanks

Other pertinant info: septic tanks pumped 5 months ago. Bottom toilet doesn't flush, backs up... sometimes gurgles from bottom toilet.

Thanks