Every time I wash laundry my toilet in the guest bathroom floods. Whats wrong with my plumbing?

My house is over 20 years old and everything is original to the house. We are on a septic system but the city checked it out when we bought it 5 months ago and said nothing is wrong with it. Every time I wash my laundry the toilet in the guest bathroom floods. It smells like a mix of fabric softener and bile. What... show more My house is over 20 years old and everything is original to the house. We are on a septic system but the city checked it out when we bought it 5 months ago and said nothing is wrong with it. Every time I wash my laundry the toilet in the guest bathroom floods. It smells like a mix of fabric softener and bile. What is wrong with my plumbing and what can I do to fix it?
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