@vilhelmmar, welcome to the forum!
Dexcom reports that several simultaneous bluetooth connections on the device create problems for the app. Did you have more than one device paired to the iphone (such as, for instance, a speaker)?
Dexcom is also aware of a couple of bugs in the app related to Bluetooth use, and recommends, as @Chris mentioned above, cycling the phone on and off once a day. We do that right before bed so that we are sure to have a good connection during the night.
You might want to introduce yourself on the Welcome thread! I am looking forward to more of your posts.