you mentioned you underwent tube ligation.., there is a less likely chance of failure of this sterilisation method since its a permanent method..even then if u feel doubtful about u becoming pregnant i would ask u to meet your gynaecologist get a urine pregnancy test done and they u would know how to deal with it..if it comes negative it might be just gastritis associated with a gynaecological problem called intermenstrual spotting ..not a very serious matter..but definitely needs evaluation and gynaecologists advice.