Sounds like they`ve dried out and shrunk, due to the house being too warm, if the gaps aren`t to wide mix some fine sawdust with wood glue to make a paste and fill in the gaps. Don`t leave too proud as it dries as hard as oak and difficult to sand down.
Go to B & Q and buy a tube of appropriately-coloured caulk and a simple 'gun' to apply it. If that sounds too technical, believe me, if a total non-handyman like me can do this - and I did for my living-room floor - YOU can. The 'gun' pressurises the caulk and, by drawing the tube-opening along the gap, you can fill it. Have a cloth handy to wipe up any excess, if the floor is to be left exposed. Simples!