Yesterday the shower drain and the garage drain backed up at the same time. 15 minutes later both drains started draining again. This morning the garage drain backed up again and at same time the toilet nearest the drain was gurgling. Once again, 10-15minutes later it drained. Shower drain did not back up this time but toilet in bathroom didn't flush all the way(different toilet). All are built on a slab. What is going on? Please help.