The rain could definitely be contributing. A septic is designed to depend upon absorption and evaporation for x amount of water based on soil and how it will be used. Too much rain and/or water use can make things like what you describe happen.
Putting some dirt may help, but not too much. You could then decrease your evaporation rate, even causing slow draining in the house because the absorption can't keep up.
And keep things out of the lake. EPA could crawl into some of your orifices pretty quick.