Period twice in one month a few months in a row. not on pill. condoms used. brown blood then period a few days later. what causes this short cycle?

Can be normal: Normal cycles vary with how long it takes the egg to mature and thus ovulation to occur. The normal range is 21 days from the start of one to the start of the next to 35 days (42 according to some sources). There is also the possibility of an inadequate luteal phase which means less progesterone is produced and the level falls sooner resulting in a period sooner after ovulation.