I have been doing lots of laundry for the past week. Lastnight all of a sudden waste started flowing in my toilets an bathtubs. When I flush the toilet water comes into bath. Took small lids off septic and I don't see standing water. What could it be?

Sounds more like a clogged line. If you opened top of septic and water did not come out.
Do you have a inline clean out

Septic tanks do not have small lids so is it possible you are opening an inline clean out? Mosst likely what Chuck says, clogged line downstream of the washer and toilet.