Yup, it's probably the rain. Geoff Fox on the ch. 8/WTNH news mentioned the other night that the ground is getting so saturated that it is affecting the "perk" (percolation) of septic systems.
When it is drier, the outflow propagates through the soil faster, so odors are less noticeable.
IANAP [I am not a plumber.]
BTW, the odor could be from manured fields, if you travel by any farmland. Don't forget roadkill. I know some skunk or something crawled into a wooded section of my street and died the other day. I can smell it, but not see it.
Kevin