Is it normal for a 4 month old female rottie to be spotting blood? I thought that the 1st heat cycle wouldn't come until 6 months or later.
Could this be a symptom of illness? She eats and plays just fine and doesn't show any other sign of sickness.
Four months is very early for a Rottie pup to have her first season, but it's certainly possible. All pups are different.
Generally the first heat cycle will occur somewhere between 6 and 8 months of age in a large breed dog, and after that there will be a 'season' approx. once every 6 months.
Some dogs will come into season earlier, and/or more often though.
I would recommend having her checked out by your vet to make sure that all is well and that this is indeed her first heat cycle though. That will put your mind at rest.
The only other thing that I can imagine would cause this is a UTI which can cause drops or streaks of blood, but generally in the urine. I personally think it's more likely that she's in season, but a professional veterinary opinion would be able to confirm this for you.