If I had two sets of reports, one from now and 1/21 and I had no clue why the score changed, then I'd start looking at the following in order:
1) I'd start with the balances. I know you said there's no change, but I'd look at the % util that might be listed on pages 2 or 3 of your report to compare that percentage. If not there, I'd look at each individual balance and make sure they are exactly the same. Even small things can throw off the score with something simple like going from $0 to $1.
2) Next I'd make sure there's nothing under dispute. A disputed account can be caused by a dispute with the CRA, a GW letter, a phone call to the OC, a PFD, etc. I'd look to make sure there are no TLs that mention "dispute". The comments on OC accts can have a big impact.
3) I'd then look at the Pos/Neg factors listed on pages 2 and 3 of your FICO reports. Look to see if any new items were added. That can have a great impact on your score. Also, reordering of those pos+neg items will impact your FICO score. I'd then look at the individual comments to make sure util (if showing) didn't increase, inquiry mention (if showing) didn't change, new accounts (if showing), new lates, AAoA (if showing), etc.
4) I'd then look at the Credit AT A Glance page and make sure there are no new added red flags. I'd look at changes in those numbers (inquiries, accounts, balances, late count if applicable, etc.).
5) Then I'd look at the Accounts page(s). Look for any balance changes, new red flags, status changes, etc.
6) Then look at each individual account. Look for any date changes if a baddie. Look at the open dates. Look at the statuses. Look at the comments. All of these can impact your score. I'd also think back to any account you just paid off or any account closed as that can impact your score.
7) If all else failed, I'd write it off as an unsolved mystery.
If one went up and one down, then I'd personally suspect rebucketing due to something aging. Most occurs around the first of each month as the account(s) turn one month older than the prior month. Maybe something is impacting you less than before (like a newly added account (within the past 12 mo.)). Adding a CC balance can impact your scores that way, esp. with TU. Maybe Mars lines up with Jupiter in such a way as to cause a score change. Who knows.
