Well, you better start taking pictures and documenting who you talk to, who you write to, when you talk/write to someone, including phone calls and 'personal visits' to the house. May is coming QUICK! IF you have a Home Warranty, read it and see what you can do. But in the meantime, do as I suggest. IF it comes to you, the judge, and the Contractor, the judge is going to be looking for how well you keep your 'documentation', certified receipts (for mailings), and so on. He's the 'judge' of who's saying the truth because in my experience, it comes down to 'he said, she said', and if you want to win, you need to BACK IT UP.
In the meantime, yes, it's not uncommon for flooring to do as you describe. However, are you willing to wait to see if the Spring/Summer brings enough 'moisture' to swell the flooring back to 'normal'? Usually, what's done is the wood is brought into the home for a few days (up to a week) to get 'acclimated'. Ask the contractor how long the wood 'sat' in the house before it was installed. Don't let him tell you it sat in the Warehouse for 2 months to get 'acclimated'. Certainly, the strips were acclimated to the Warehouse; NOT your home. What's the make, model, serial # (well, not the serial #) of the flooring? What do you have underneath (all layers)? Was it nailed or glued or stapled? And is there a basement or crawlspace or concrete slab below the floor?
Answer these questions and we can go from there. There doesn't seem to be a moisture problem (otherwise, you'd be writing to us about that and not complaining about a shrinking floor ...)