Longer than expected background check

Hi all,

I got an offer contingent on my background check results at one BB bank. It's a junior equity research role. After providing my compensation at current company, my info was sent to go through background checks. I've been waiting for the results for almost 5 weeks and started to feel anxious. I'm always 100% honest on my CV and don't have any criminal history. However, there's one specific internship experience I'm worrying about. 

I interned at a sellside research in my college days. It was an unpaid summer intern role and that company got acquired by another bank some time after my internship ended, so I've lost contact with my supervisors at that company. I did receive an internship certification from that company, but the BB HR lady didn't ask for any reference of my past employment, so I didn't send it to her in advance.

I'm afraid that the background check company might not be able to find proof of my short employment (unpaid) at that company (acquired), and that the HR lady would perceive me as a liar. Plus the background check has taken longer than expected. Am I in trouble? Feeling anxious.

Thanks!