I had silicone implants a year ago. What's causing the fluttering that I keep feeling?

If the fluttering sensation comes along with a sensation of heart flutters, breathing changes - especially shortness of breath, dizziness, or fast pulse, you need to see your primary doctor immediately.  Assuming that the "fluttering" that you are feeling is something associated with the breasts themselves and not something else, there are a couple of thoughts I have.  I is not unusual for the pectorals muscles to have some movements like spasms, called fibrillations or fasciculations, especially after vigorous activity.  Fibrillations are generally very fine movements within the muscle, while fasciculations are coarser movements of the muscle itself.  They are generally not painful, but they can produce a sensation of movement, or "fluttering" as you describe.  Things like salt depletion in the body's system, or other chemical imbalances can produce these things too, but for something like that to happen, I would expect other muscles in your body to be involved too, not just the pecs.  Another thing that can happen is actual movement of the implant itself as it slides through the pocket and over any irregularities of the chest wall, such as the ribs or any imperfection in the capsule.  This can cause some vibratory movements which can feel like fluttering too, and they are more likely to occur when you are changing positions or using your pecs actively.  These are a couple of thoughts that I have, but the best thing I can tell you to do is pay a visit to your surgeon so that he or she can do a more detailed history and examination of the breasts to try to pinpoint exactly what is going on.  Good luck.