You have one of two problems,
#1 you said it was raining you might have overloaded your septic tank,
when you do not run water in the house for a couple of hours then the septic tank drains down,
when you run a load of laundry you fill the tank,
you need to first check the outlet filter on your septic to see if clogged then if that's not it you need to have a plumber or septic pumper
out to check tank !
#2 if you are on a city sewer then you have a blockage in line see above about draining down works the same way on sewer line
and you need to call a plumber / rooter cleaning company a. S. A. P. To clear before it backs up completely