Blood in toddler’s stool? Very panicked mom

So my toddler has been constipated on and off for a while now. Im trying sooo hard to get him to drink water and he does but not enough, I know that. So hes been constipated for awhile. Last few weeks hes been really good actually, been last few days his poops have been pebbly and hard so I know hes constipated again. But today there was some blood and im FREAKING OUT. His ped isnt in today and everything is closed so im sitting here panicking

Ill definitely take him in when they open but someone please calm me down. Can it be from constipation and pushing? I also started him on cow milk recently so could be allergy i guess. Anyways, imm just so scared and sitting here crying until the ped gets in