Loud Pop in Breast Implant

Most likely this was your capsule rupturing. The best way to tell for sure is a MRI if your plastic surgeon feels it is indicated.

Although it is possible that the implant ruptured, it is unlikely. Implants can withstand alot of pressure without loss of integrity. However, an MRI is a good option to evaluate the status of the implant and capsule.

It is not possible to know exactly what happened without more information and a physical examination, however, you narration suggested that you boy friend performed a closed capsulotomy on your breast. Closed capsulotomy is an old technique to soften a breast with capsular contracture. Closed capsulotomy has been largely abandoned in the US, because it can lead to uncontrolled bleeding, rupture of the implant and malposition.

Followup with an MRI is important to check your implant. Mammograms and ultrasound are not as reliable as the MRI. An MRI is about 90% accurate (so wrong 10% of the time), but it is the best nonsurgical way to check an implant currently available. The only 100% accurate method is to look at the implant directly in surgery.

An open capsulotomy or capsulectomy is the preferred method of treatment for symptomatic capsular contracture. Both require surgery, and allow for precise modification of the capsule's scar and direct inspection of the breast implant.

When a patient experiences sounds from the breasts, often it is
early in the recovery period from breast augmentation surgery.  This is normal and is a reflection of some of
the irrigation fluid that has remained in the breast pocket.  Over a short time this irrigation fluid
becomes absorbed, and the sounds go away. 
Your situation is different in that the timing is 13 months post-op.   Fluid in the breast pocket is still likely
the reason for these sounds. However, newly accumulated fluid in a breast
pocket over a year after surgery is called a delayed seroma and should be
investigated.  A breast ultrasound can
confirm the diagnosis of seroma.  Delayed
seromas are associated in the majority of cases with having textured gel
implants.  Delayed seromas can also be
associated with a rare implant-associated cancer called anaplastic large cell
lymphoma.  I would suggest following-up
with your original plastic surgeon so that he/she can evaluate you for these
new symptoms.

Silicone implants are designed to be able to withstand significant amounts of pressure, and the loud pop sound you heard may be due to a rupture of the capsule around the implant. An MRI is better suited than an ultrasound in assessing whether the implant has ruptured. Make sure you follow up with your plastic surgeon afterwards to discuss the best course of treatment, if any.