This is normal this time of year, air expands with higher temperatures, so naturally it contracts when it gets colder.
I just had to "top up" my Swasticar tyres as well, they were suddenly down to 2.5 all of them, so i drove where i needed to go (Short trip) and when i got to the pump, they har risen to 2.7, so i pumped them up to the recommended 2.9 and the error went away.
Outside temperature had dropped to 8c, so it was expected they do this, i always try to keep them inflated to whatever they need to be in the summer, if i had inflated them to 2.9 bar in 8c, they would be well over 3 on a hot summer day, driving fast on the motorway.