Would sleeping on my side cause symmastia? (photo)

No, sleeping on your side shouldn't cause symmastia. If you are concerned about developing symmastia, please see your surgeon for a better assessment. 

Symmastia is a condition which can occur when the implants are too close together. Patients often refer to this as a “uniboob.” This can occur for the same reasons as displacement noted above, but in this case the pocket(s) is/are too far towards the center.Correction of this problem may involve using different implants such as textured, a smaller size, or a smaller base and placing the implants in a new pocket. On occasion, this may require surgery in more than one stage to allow the tissues to heal and then place the implants. If the implants have been placed above the muscle, converting them under the muscle frequently will help and, again, the use of a dermal substitute may be required for additional support if the tissues are thin. The techniques of capsulorrhaphy and neopectoral pockets may apply here as well.

Your breasts look great and should not be affected by sleeping position. You might have worried about sleeping position earlier in the post op course but no need to worry now.