get a sample in a clean container if you can - mid flow is best - then you can have a look to see if it looks odd or discoloured - if it does nip a jam jar full down to your vets and they can do a quick dipstick test on it to see what's going on - won't tell you loads but will at least tell you one way or the other if there's blood in there.
Could be loads of things and I thought my mare as passing blood because of the colour of the wee patches until I realised I'd swapped onto redwood shavings and they go pink when peed on -