Your strings have gained tension due to the change in temperature and humidity causing the wood of your guitar to expand and contract. Metal strings don't expand or contract enough to cause detectable tuning issues alone. The wood of your guitar, however, does. The truss rod does the best job it can for keeping your neck at the right angle relative to the body, but it's not perfect. A slight movement of the neck backwards (towards the z axis relative to the body of the guitar) will cause your strings to go sharp enough for your ear to discern. You can test further this by playing open strings and pushing on the headstock in different directions perpendicular to the body of the guitar--just don't push too hard. As relative humidity changes, the wood of the body and neck of a guitar will expand and contract in two ways: tangentially and radially. Tangential movement lies parallel to the growth rings of the wood while radial movement is perpendicular across the growth rings. Another thing to consider is that different species of wood are more hygroscopic than others. This simply means that one species, take Maple with a radial growth coefficient of 0.00353 for example, will absorb more water--and thus move more--than another species like Cherry that has a lower radial growth coefficient of 0.00248. So, while the neck of your guitar gains or looses moisture, the wood will expand and contract relative to one of the two directions I explained earlier. This movement in either direction can very well be enough to cause more tension on the strings, thus bending them sharp.
A great article explaining all this a little more lives here. It's about cabinets, but very much applies to anything made of wood because the physical properties are universal.
Also, another list of some common woods and their growth dimension coefficients is here. According to that book mahogany is on the lower end of the spectrum, so it doesn't move very much. However maple, a very popular wood for guitar necks, tends to move a lot tangentially (parallel to the growth rings, or grain of the wood) and consequently is a little more susceptible to changes in humidity.
So there's the physics of it :D. In the end, the environment your instrument lives in is very much an factor to it's overall health--especially if it's made of real wood.
Also, see the answers on this question for other ideas as to why your guitar goes out of tune (usually).