I am very new to Starcraft II: Wings of Liberty, and I just purchased the game. So after purchasing few things from the armory, I had around 30,000 credits left, and I saved&exited the campaign. Then, when I entered the campaign again to the ship, I had over 100,000 credits! So, I saved, exited the game, and entered again, only to see that I now have over 200,000 credits. It looks like that every time I enter campaign, I get a lot of free credits. This is obviously a good thing, but what is happening? Is this going to slowly take real money from my credit card? Is this going to end?
The campaign for Wings of Liberty does not normally award credits at random. Credits are awarded on mission completion, sometimes on bonus objectives, and a little bit more once your research is topped out and you bring in Zerg/Protoss samples you don't need.
The total number of credits possible in the campaign is fixed in fact, and limited enough to not allow you to purchase every available upgrade before the campaign ends. Multiple walkthroughs have been created around this fact to point out more optimal upgrade options.
So this would either be a bug, or you are simply loading different saves at different stages in your campaign. You mention purchasing and then reloading - the campaign doesn't autosave in the HQ screens last I checked, just on checkpoint or end of mission. So you would have essentially lost your purchases, that's be another explanation for your extra cash.