Clearly, the most likely explanation is hormonal due to the cyclical variation. You did not state your age but I have heard this complaint quite frequently from women in their mid 40's who are perimenopausal. Not infrequently they have had enlargement of their breast tissue over pre-existing implants of long-standing duration and also commonly report 5-10 weight gain.
I don't know the exact significance of these findings but they are a common grouping of symptoms/complaints.
There is an old expression in medicine: When you hear hoofbeats outside your window, it is most likely a horse and not a zebra." Well the above explanation is the most likely the horse. There are many zebras the second most likely is implant rupture. In the US, the FDA recommends MRI evaluation 3 years after implantation. You state that your implants were placed 8 years ago and it is wise to contemplate an MRI if you have not already had one. Many of the other potential causes have been spelled out by the excellent insights of my colleagues below.
I hope this helps.