What Causes Lump On Thigh Before Periods?

hi, i am 19 year old female, everytime my menstral cyle comes a day before i get this large lump on my inner thigh, it is very painful, for the first day of my cycle, then it fills with fluid like a pimple, i popped it, then it is no longer painful, then once my cycle leaves, so does this lump. what should i conclude this is?