Breast looks like it slides to the side (toward armpit) when laying on back (Photo)

Congratulations on your breast augmentation.  The discomfort that you describe is called lateral displacement and occurs when the breast implant pocket extends too far to the side.  This can be corrected with a surgical revision.  It is best to discuss this in person with your plastic surgeon.  For more information please read below:

Thank you for your pictures. The pocket that implants live always seems to get a little larger allowing breast implants to drift somewhat toward the armpit when one lays down. Your implant position while lying down looks reasonable. Natural breasts that are reasonably sizable also do the same thing when one lies down. I would not recommend any treatment for your concern.Best wishes.

I am sorry to hear about your concerns after breast augmentation surgery. You demonstrate your concerns nicely. Although some “falling to these sides” of breast implants is quite normal when you lie down, if this occurs “excessively”, it may be a concern to patients. This phenomenon is called lateral displacement of the breast implant. Surgical correction is an option.
Generally, the lateral breast implant displacement can be corrected using an internal suture technique, decreasing the size of the pockets and moving the implants toward the midline.   In my practice, I use  a 2 layered suture technique to close off the space laterally (towards the axilla). The use of acellular dermal matrix is an option (although not usually necessary)  especially if significant implant rippling/palpability is present.  
 My best advice: select your plastic surgeon carefully. Make sure that he/she has significant experience achieved the types of outcomes you will be pleased with. Then, communicate your goals carefully, as well. Working together you will come up with a good plan to achieve your goals. You may find the attached link helpful to you as you learn more.   Best wishes.