Welcome to the forum.
What you're experiencing is quite common in cold temps.
The normal culprit in this situation is there's condensation/moisture in the hydraulic system. Moisture is pretty normal since it's a hot then cold cycling of the system from when you run it and when it sits. That condensation may have created small ice particles in the system and is causing temporary blockage in the sump filter until it melts.
Don't know how often or how recently you change your hyd fluid and filters but if it's been a long time, maybe you're due. Fresh fluid & filter won't have those little water particles in it. Make sure you use OEM or OEM approved fluid and filter.
In the very cold temps, I let the tractor warm up for at least 15-20 minutes to address this very issue. Sure, the engine is warm but since they hyd systems is more exposed to the outside (think hoses, cylinders...), they take much longer to warm up.
A couple tricks you can do to get your hyd fluid warmed up faster.
Turn on the PTO (assuming no implement attached).
Put your loader control in float mode.
Both of these things will get hyd fluid flowing through the system faster