RASH ON LAB (CLUSTERS)

I suppose it could be from soap,,, or is it possible that where you live the grass got fertilized, or weed killer applied, and your lab was laying on the grass? Carpet cleaner or something new in your home?
I'm sure others will  be along for suggestions as to what you can do for him now,,to try to rid him of the rash.

It could even be a specific plant that he brushed over when he pooped or something.  When they crouch they are in a position for a time and may return to near the same spot.
 
Can you get an aloe plant??  (don't use aloe cream from the store -- you don't want any additives that may irritate further) -- if you cut the sides off the 'leaves' and then split them lengthwise -- just scrape the goo from the inside.  If you do it on a piece of plastic wrap you can squeeze the goop into a small container.  Apply a couple of times a day and refrigerate.  SUPER good to sooth a rash and heal it.

This is what I found out at the vet today. My girl had large bumps on her back plus a small rash on her tummy and legs. Kinda like what you describe. I was about 1 week late giving her Advantix so she had a opportunity to get bitten the past couple of days. They vet said that this time of year the fleas can have a more potent bite then in the winter and it makes some dogs break into hives. They gave her a benadryl shot and said to make sure there is never a in between time of her receiving her fllea meds but if it happens again I can always giver her two adult sized regular regular benedryl and it will relieve the itching, bups, rash, and discomfort. So even though you can't get your fur baby in for a week maybe you could try some benedryl!

NOOOOO Not "non-drowsy" -- that would be decongestant and that is too much decongestant. 
 
You want plain old Benedryl (diphenhydramine).  It will make the dog sleepy but that eventually goes away.
 
The dose is 1-2 mg/lb body weight two to four times a day. IF you are seeing hives then give the 2 mg/lb dose. 
 
The over-the counter pills are 25 mg. each ... just make sure it is PLAIN Benedryl -- not with any other drug in it (no severe cold formula or non-drowsy -- those have other things). 
 
You don't want sudafed or acetaminaphin in them AT ALL.  They do give sudafed to dogs but it's for a different purpose and a different dose ratio.  But aceatminaphen (Tylenol) IS POISON to dogs so make sure you have the pink/white capsule of Benedryl (or if it's caplets they are pink). 
 
Call your vet to make sure -- but this is an important difference and I don't want you to inadvertently poison your dog. 

thanks for clearing that up calliecritturs, I had just heard what the vet said this morning but didnt read what it said on the paper she gave me....oh I woulda felt horrible if she took the wrong kind, I'll go amend my post right now!

Is it poison ivy?  My dog got a bad case of it once on her belly and inside rear legs.  I rubbed on her the human cortisone creme after I washed the area off with cool water. Never wash poison ivy off with warm or hot water.  It went away in a week.  Plus wash the bedding in hot water.