Ok I would try some spots outside of Boston but nearby, I see some other recommendations in the thread that you have like Tufts, so go with those. I would add in others outside of Boston proper, BIDMC Milton for example may be able to accommodate you sooner than the main campus (assuming they're only checkin at main if you're calling main). Definitely go to the ER if it happens again, and if it's possible to speak with that Amherst PCP again I would maybe try to confirm you can wait a couple weeks, or if it's something that needs to be looked at in terms of days.

Definitely start the process to get a PCP here in the city as soon as you can, for example I have a BI associated PCP & I get referred to people within the hospital to get appointments rather than just a general referral. Will be way less stressful in the future for any kind of medical treatment including annuals physicals with a PCP.

Good luck and hope it's not too seriosu!