My left breast implant feels like it is popping out from the muscle. Is this possible? Should it be replaced?

I would encourage you to schedule a visit with a Board Certified
Plastic Surgeon immediately to determine the exact source of your
discomfort.  You will certainly require a
clinical examination and may also require additional imaging studies such as an
ultrasound, mammogram, and/or MRI.  If
these indicate a rupture, your surgeon will likely encourage you to proceed
with surgery.

If you have a smooth surfaced implant, it won't be attached to the tissue. The pocket that the implant sits in may be stretched out and the implant is sliding in the pocket too far to the side.

It sounds like you have an implant malposition problem.  With the implant falling off latterly when you lay down. This is due to it overstretching of the capsule. It is not a muscular problem. The surgical correction this would involve a capsule pixie or narrowing the lateral margin of the pocket.

Please seek IN PERSON examination of your implants to see if displacement has occurred. See your surgeon soon.

This is something your surgeon will have to see in person to be able to make a reasonable assessment of the situation.  If the implant pocket is too big you may need a revision to control the position of the implant.