Credit score plummets for no apparent reason

I have a credit card that, through their online service, gives me a Transunion credit score. I've heard the score given through these types of services is not exactly accurate, but it's a complimentary service and sort of act like "credit monitoring".

Now, I'm in a position where I may have to pull my full credit report, despite having just pulled it in February. My score has been around 790 (not great, but acceptable) fairly consistently, yet just this month the new score was reported as 755. Now, I know I just opened a credit card (which I am unfortunately closing since I missed the part where my rebates could only be used for one or two things - things I don't have use for), but that certainly should not account for a 35 point drop.

Most interesting is when I clicked on a link that gives me a generic description of why my credit score is such and such, it has 2 items. The first is that my most recent account is very new. Check. The second is that I have multiple accounts that show late payments or derogatory marks.

Huh?

My only guess is that some company I do business reported a payment as late. I know it's not my credit cards, because I have full account access and can see when payments are posted, and I haven't paid an interest charge in years (my one mistake a couple months ago with making a short payment, it turns out, I was looking at when the billing cycle closed, not when it was due, so I just paid the remainder a day later and of course no interest charge - I was surely having a brain fart that day).

So that leaves me with utilities and a few services. Home security alarm, maybe Sirius radio? I can only think of 2 possibilities.

1) My electric company one month didn't send me a bill. When I knew it should have been due, and didn't have a bill in hand, I called them up and asked them where my bill was. Turns out, they said they mailed it but it never arrived and I was one day late at that point. I had them reissue the bill, paid it, and they removed the late charge. Could they have reported me late?

2) Sirius radio. I have a subscription that automatically deducts from my credit card every 3 months. Out of the blue I get an email from them saying that my account was past due. Confused, I called them up (using the number on my paperwork, not through the email which I immediately suspected was a phisher) and for some reason they just didn't charge my card. The card number was the same, the expiration date was the same; they just decided not to take any money and decided this was my fault. After "updating" my card from the old number to the...same number, they charged it correctly and all was well. Could they have reported me late?

That's all I can think of. Well, I suppose I need to shell out the cash to pay for the report and see just what's going on.