This or something similar has happened to me recently as well. I searched flights, then searched alternatives, then back to the original search and they are more expensive, all within the same session. Then I found that if I cleared my computer cache and cookies and then tried the search again the lower proce was back. I was not logged into my account then---when I have done it logged in, searched, and then gotten the higher price when I return to the original options, and then got the lower price logged out.

It seems the co.com was using the cookies to know what I had searched and then raised the price when I returned to that route. Of course, it could have all been coincidence due to other customer's actions.