I had a cbc test &everything was in normal rage except my platelet count which was 687, my doctor called today saying it is now higher.. is it cancer?

Not likely: While your platelet count is high, it is very unlikely to be cancer. Even a subclinical deficiency of iron may cause platelet elevation. You did not provide the actual results for your CBC which would have helped. All laboratory results need to be interpreted in the clinical context and the doctor who ordered the tests is usually in the best position to do that.

Unclear but unlikely: At age 20, it is unlikely this would be due to cancer. A platelet count goes up for many different reasons including inflammation in general (infection, post-operation, auto-immune disease, etc), iron deficiency, or absence of spleen. To read more about it the term medicine uses is "thrombocytosis". Likely, your doctor will either evaluate for these potential causes or refer to a hematologist.