Recent cbc: rbc 4.48, hgb 11.8, hct 38.1, mcv 85.0, mch 26.3, mchc 31.0, rdw 15.8, plt 503. how "abnormal" is my plt in relative terms? what is likely the cause of high plt (poss iron deficiency?)?

Increased platelets: Your platelet count of 503 is technically high, but not extremely high by any means. You are also mildly anemic. This anemia is most likely due to iron deficiency as this is the most common cause by far in young women. Further tests would confirm this if necessary. And yes, iron deficiency is ONE cause of a high platelet count. Continue to sort this out with your doctor.

Check for low iron: Your platelet count by itself is not worrisome, but elevated platelets with an elevated RDW (the spread of Red Cell sizes) is often an indicator of iron deficiency. Iron deficiency is common in menstruating women, especially vegetarians, and can cause fatigue before it is severe enough to cause anemia. Ask your doctor to check your iron saturation level, and consider a daily vitamin "for women."

Repeat test: probably of little import; redo blood count in a week+.