Old period blood burning

So I'm coming towards the end of my period right now and I'm having old blood come out (it's a very dark brown, almost black colour). Since the old blood has been coming, every morning I have been getting a burning sensation in my vagina which is only relieved once I pee, and clean/wash myself and get rid of the blood down there which might not have come out on to my pad because of the fact that I've been laying down. Is this normal? Is my blood really acidic, or what's the cause? I couldn't find anything on the Internet so here I am! Thanks in advance for your help x