change in how often I am menstruating

I am a 27 year old female. I have been menstruating from the time I was 12 yrs. and 4months old. I will be turning 28 a little over one month. I have always been extremely regular, right from the time I began menstruation. I have never had children, or been pregnant(as far as I know). I menstraute every 21 days, and I bleed for usually around 4 days. The past three times I have menstruated it has occurred at around 14 to 17 days. The last time I menstruated, (I am currently menstruating now) I bled for about 3 days, then 2 days later, a little bit more blood came out, not very much. My bleeding seems to be just slightly lighter than normal. Also, I am not certain, but my breasts seem to be slightly fuller, although that could just be due to my menstrual cycle, as they do swell around menstruation time. I do not think it is very likely I am pregnant, as my partner had a vasectomy years ago, and his tubes were cauterized so that they would not heal back up. We have been together for three years, so I do not think it is likely they would suddenly heal now. This change in my cycle has started about 9 weeks ago. Is it possible my cycle is changing due to my age? I have no way to go to a doctor or clinic as I do not drive or have transportation, nor can I afford to purchase any kind of pregnancy test. I understand that any answer I receive will not be a diagnosis, but I would just like to have a general idea of what may possible be going on. thanks