I can't tell you whether or not you are actually losing or gaining, but I can say for sure that Mint's Investment tab is FULL of known bugs and should not be trusted. Go to the source it is pulling and verify the info is correct. If it doesn't match up, then Mint is wrong, not Fidelity, for example.
I have been told before on Mint that my brand new Betterment IRA account had lost over a million!
I have also noticed that if you've only recently linked an investment account in Mint, then it screws up any view that is longer than the data it has access to. So for example if it has 3 months of data and you click to see a year's performance, it will show crazy red losses regardless of what's actually happening.
Use Mint for everything else, including reliable investment totals on the Overview tab, but don't ever look at their investment tab. Go to Vanguard or whoever's page instead...and don't peek too often anyway ;-) .