We have a problem with a really bad smell that comes from only our shower drain if we use the shower while it's raining. It doesn't matter if it's light rain or pouring, if we use the shower then a sewage smell comes out of the drain. Then the smells last a day or two before it goes away and it only happens with the one shower. What could cause this? Please help, I thought it could be a septic tank issue, but then again that would cause all the drains to smell and not just one, right?

I would start with the basics. Although the part about the rain puzzles me this could be sewer gas blown back from a partial blockage in the drain line.
Trey this!
Snake the shower drain followed by a large pan of boiling water and capped off with a few quarts of bleach. Get back to me with the results. Good luck, Tom

Thank you I will give this a try and let you know.

Did you put this in or was it already in? It sounds like there is something wrong with the trap. That would be the only thing that would let a smell up through the drain

That would be the only thing that would let a smell up through the drain Not quite so! While losing a trap seal will allow sewer gas to escape that's quite rare. Traps very rarely go bad by themselves.
A more plausible explatintion would be a partial blockage. With a partial blockage the discharge runs down the drain line, hits a clog and bounces back sending a bubble of sewer gas ahead of it, Then the discharge slowly drains away. Regards, Tom