M.; yes stress can change them and make them dissappear totally, it also can mean you can be pregnant, although the tests did come out negative, ive seen my sister in law take tests, shes not pregnant , she is, shes not, she is, this went on for months, till finally she was, like 3 months later, she was 3 months along, its crazy what our bodies will do, i would definitely go to a dr if you dont start one soon, sometimes our stress will make it so you skip amonth ive done this many times, it could be too, that your body is resetting itself so to speak, you probably are just missing a month and will start soon , or a week or month later, when our periods do strange things we do need to get it checked, i would wait a week , and then go to drs, ask your freinds who they see, they might recommend a good dr, either way dont stress, and do some reading or studying and redirect our mind from the stress, enjoy life and be happy, D. s