This could be a sting / insect bite. In which case I'd suggest giving him 100mg of Benadryl (4 of the 25mg tablets) by mouth every 12 hours until he seems normal again.
A snake bite would be more concerning and likely necessitate further supportive care. Wounds can be hard to find, especially without shaving fur. This is the most concerning possibility. Depending on the snake, he could develop more serious complications related to any toxin in his system (e.g. vomiting, diarrhea, shock, etc.).
I also am wondering about something called "swimmers tail". This is an acutely strained muscle in the base of the tail. It is classically severely painful, they will NOT lift their tails up (or cry out horribly if you try), and are of sudden onset. As the name implies, it's sometimes associated with dogs swimming (the thought is the combination of jumping into the water or turning in an odd way along with the cold water sets them up for pulled muscles in their tail)...but any vet that has seen them will tell you they've seen them with no recent swimming. Also, I've seen more of these in labradors than any other breed BY FAR. The treatment is anti-inflammatories, muscle relaxants, other analgesics as-needed, and rest, and they do fine. I don't recommend giving human anti-inflammatories (no ibuprofen / tylenol / aspirin etc.).
Another possibility is similar to the tail injury - a slipped disc in his lower / lumbar spine is possible.
I'd suggest giving some benadryl as it certainly won't hurt and may help him rest, and get him some comfy bedding to rest on (kennel if necessary) to keep him quiet until you can get him in to your local vet. If he seems to be getting worse via signs of systemic illness (vomiting, diarrhea, abnormal gum coloration, progressive weakness / lethargy) then you might want to get him in asap given the snake bite concern you mentioned.