SOMETHING HAPPENING TO ME? something weird has been happening lately, i will just be doing normal things, watc?

SOMETHING HAPPENING TO ME? something weird has been happening lately, i will just be doing normal things, watching tv, doing homework, sitting down e.c.t and all of a sudden my heart will really really quikly start beating really fast for about 5 seconds and my head will shake and i will feel weak. i told my mum she said im just being silly. whats this a symptom of? im 15 x