Your breasts certainly undergo changes related to your pregnancy, and enlargement is one of those changes. Without additional information or examination, I can't state this with certainty, but I would suspect that both breasts are enlarged somewhat. The fact that your right breast hurts and your left doesn't is the real reason for concern--if both sides were sore you would undoubtedly feel this is just "pregnancy-related" and not worry or ask.
But with one side symptomatic, I understand your concern, and can only offer some general comments without an examination. First, as your breasts swell with pregnancy, the scar capsule around your implants may be stretched, and a nerve or nerves within or nearby the scar can be also stretched, sending pain signals, especially when you raise your arm. Of course, each breast capsule may irritate or stretch different sensory nerves, and that is why it is not unusual at all for one side to hurt and the other to remain asymptomatic!
Without breast-feeding in the equation (yet), mastitis is less of a concern, but still a possibility. That can happen on one side only, and examination by any qualified plastic surgeon or even a GP who is a bit more, shall we say, engaged in the care of her patient can determine if this is an issue.
Capsular contracture can also cause pain, but as my colleagues have stated, it is much more likely that your pregnancy is the issue here, rather than any problem with implants that have been "just fine" for the past 3 years. Seek consultation with a qualified local plastic surgeon and your concerns can be addressed. Good luck with your new addition and Best wishes!