"Already wormed" is actually a meaningless statement.
The reason is you could have used the wrong drug, the wrong dose, the wrong frequency, or given for the wrong amount of time ... which means there's a real chance your dogs aren't "wormed" at all.
I agree with Evo, it's either worms, coccidia, or even some kind of virus (does not sound like giardia). But, for some reason, it sounds like blood in the intestines/colon.
What kind of wormer did you give, how much, and for how long?
There is also an abstract virus going around, that makes dogs shit "blood pudding" (or black-blood-tar) for a few days, but goes away (although some dogs die from it).
Hookworms and whipworms also produce a similar plight, and you have to use certain drugs (at certain doses) to get them ... "regular worming" with pyrantel or ivermectin will NOT work.
Good luck,
Jack