Just got my first credit card after having issues in 2013, a Capital One Platinum with a $3K credit limit. I have been paying it off early and in full for 4 months now, and checking CK a couple of times per month.
Well, this month I had a huge surprise, my credit scores for Transunion and Equifax both dipped more than 100 points each, going from ~740 to ~625.
When I click on "See what's changed," I get this for both reporting agencies:
We found no changes to your Transunion credit report.
We know it can be jarring when scores change but you don’t see why.
A few score factors are more 'invisible' — like an account getting older
or a scoring model adjusting its recipe.
Expecting a change that you don’t see?
Transunion may not have updated your report yet.
So what the heck happened?!? 
Hi Stevie, welcome to the forums!
Did your FICO scores drop?
Credit Karma scores are of the Vantage 3 model and respond differently to changes to your overall credit than FICO; it is common to see a drastic difference (higher / lower) between your FICO and Vantage scores (Vantage scores also tend to rebound from unexplained drops fairly quickly). FICO is what lenders use to evaluate your credit, not Vantage. CK is good for monitoring changes to your reports, but the scores may be ignored as they do not factor into credit decisions. You should be monitoring your FICO scores -- be concerned if you experience a 100 point FICO drop.
Monthly Experian FICO 8 score (FREE): https://www.discover.com/free-credit-score/ -- You do not have to be a Discover cardholder, just sign up.
All three FICO 8 scores: $1 / 7-day Trial at creditchecktotal.com -- Just be sure to cancel before the trial ends. You can do this more than once (at least, until they catch on that folks abuse this...).
Experian.com offers a variety of FICO monitoring memberships. The most expensive is CreditWorks premium for $24.99 (or $29.99, not sure) / month. That gives you daily Experian reports and FICO scores with monthly updated Transunion and Equifax reports and scores. Experian also runs creditchecktotal (mentioned above) so if you do the $1 trial, it will give you access to the Experian mobile app & website which is exactly what you'd get if you went directly through Experian.
Some card issuers also offer monthly FICO scores -- like Citi, Amex, Discover... -- so you should check with your card issuer to see-- just be sure to check the small print for the scoring model they are providing becuase many offer Vantage scores because they're cheap to provide.
myFICO is the most costly because it provides all 28 versions of your FICO scores, not just FICO 8. The other versions include mortgage, auto, bankcard enhanced models, as well as FICO 9.
