erythrocytes are red blood cells. So it's the same as what you already know. Don't start freaking out - it could be from an infection, it could be from a small kidney stone, it could be from your period (when did you have it when you got the test, or when was it due?) I had some of that when I had a urinalysis about 7 days after my period.
Now my friend had been having bleeding on and off for 2 years - and I mean REAL bleeding, not just a few microscopic cells.... she would actually pee red!! They found NOTHING wrong! Many times that COULD happen and the reason is never found. She did have 2 kidney stones which the doctors said is the reason (the stones dissolved or something, they are gone now and so is the bleeding)
It's NOT an odd thing to happen.