It's totally normal. Strings are made of metal which will expand and contract with temperature. You tune the strings when they are "cold". you play a few tunes and the strings warm up from your hands. You re-tune the strings while they're warm. Everything's now fine. You sit the bass down for a while and the strings cool off by radiating the body heat they've absorbed into the environment and they contract. You pick up the bass and lo and behold the strings are sharp. You tune the strings when they are "cold". You play a few tunes and the strings warm up from your hands. You re-tune the strings while they're warm. Everything's now fine. You sit the bass down for a while and the strings cool off by radiating the body heat they've absorbed into the environment and they contract. You pick up the bass and lo and behold the strings are sharp. Rinse and repeat as often as necessary.
mech
Do you really find those changes are notable (pun)? How much appreciable change in pitch can be attributed to those factors?