Get MRI of breasts three years after getting breast implants.
Hard to tell what's causing this, and probably unrelated to breast implants. But I would recommend an MRI of the breasts every 3 years in any woman with silicone implants. And get a good exam.
If you are having alternating warmth and pain around your implants, you really need to see your surgeon to make sure there is no suspected infection or other problem occurring.
If you are feeling warm implants and tender every two weeks now, I would suggest getting them evaluated by your doctor.
See both your gnecologist and plastic surgeon
Your symptoms, which are in both breasts, are more likely unrelated to the implants. You should have your GYN doctor examine you and look at a recent mammogram. Your plastic surgeon should examine you and see if there is any evidence of capsular contracture or implant rupture. Given that it is in both implants, it is unlikely to be rupture. Good luck with your care.
Have your implants checked
It is unlikely that your implants are infected. They could be leaking, but the cyclical nature of your symptoms suggests something with your menstrual cycle. I suggest a check up with your gynecologist to start. He/she can refer you on from there if need be. Good luck!
Have your implants evaluated
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.
Your symptoms have many potential answers - indolent infectious process, implants leaking, menstrual cycle or another process that is unrelated to your implants. You should see your plastic surgeon and gynecologists for an evaluation. You will likely need blood work, and radiologic studies such as an MRI prior to a diagnosis is made. You should see them ASAP. Good luck.
A clinical exam will be needed to rule out any developing capsular contracture, if implant leak is suspected an Ultra sound or MRI might need to be done. Infection is another differential diagnosis, but it is unlikely in the abscence of other clinical signs.