Is it possible sewage from beneath the ground has seeped higher due to increase thawing thus flooding as the ground water begins to saturate the surface ?
Having had to replace an entire septic field due to broken drain tiles or poorly installed draining can be a real chore not to mention expense.
I hope the best for you and please let us know if you ever do determine the source.
If I can suggest a home remedy for the stench- Using four small (6-10 oz ) glasses or jars and ordinary wine vinegar.
Pour about 1- 1 1/2 to 2 inches into each of four "cups" and place these, one in each corner, of the room that reeks. Allow at least an hour prior to checking results.More time may be required but this is usually about 2-3 hours max.
***We have a rental that apparently a stray cat had found a way into but never left before death .When we opened the house after being vacant for a month the smell was horrid. I tried the solution suggested and found it worked very well. ***