Oh Sam - I hope he's ok.
I had a simillar scare with Harry a couple of weeks ago. The morning after my husband had left on a work trip for 10 days, Harry had very very runny poo, going about five times before he started passing blood. Like you I rang the emergency vets and was told that if he is 'ok in himself', which he was, then the blood could be due to 'over straining' rather than being anything worse. Harry is 15 months old (so older than your pup), and I was told to give him a bit of scrambled egg and rice, and maybe a little bit of chicken mixed in to help settle his tummy. If he wasn't better in 12 hours I was to take him in. But he gobbled up the rice and chicken (what a surprise!) and his tummy settled a couple of hours later. He did look quite sorry for himself for a day or so, but the new rice, egg and chicken dinners seemed to make him happy. I gradually introduced his normal food again over the next 48 hours.
Doggies can get minor stomach upsets and its no surprise when I think the revolting things Harry attempts to digest - I do hope your puppy just has a little upset tummy and is well again very soon! It is so worrying when your baby is ill!
And just to illustrate my story - Harry has just vomitted a small pile of sticks and grass at my feet!