Yeast infection?

Every time I have sex, get fingered, or eaten out I get a yeast infection within the next day or two days later. Pleaseee help me I don't understand. I keep myself very clean and I have no problems unless I do something sexual like that. Please give me tips on how to get rid of a yeast infection also, I have no way of getting medication because my mother doesn't believe somethings wrong and I have no other way of solving this problem(I'm 17 and don't have my own car yet.) Please give me advice.