Why do I have clumps of grass in my bathtub after a sewage overflow?
In many small towns, or towns that used to be small, when the sewer system was constructed the people in charge decided to save money by connecting the storm drain system into the sewage collection. The idea is that all the rain water will be collected at the sewage facility and treated together. What usually happens after the sewage handling has been outgrown by the population or there has been a very large rain is that the sewage system will backup into the rain water collection and causes problems. What I suspect has happened in your case is that people mowed their lawns and the grass clippings ended up in the rainwater collection drains. The rainwater emptied into the sewer system and then the system (unfortunately) backed up into your house. I don't think you have more to worry about past the fact that your town has a sewer problem. Though I probably wouldn't eat any  fish you catch nearby.