Ah, Spring! That magical time of year when a young man's fancy turns to love... What? It's still winter? That miserable time of year when a young man's suspicions ruin a perfectly good relationship? Ugh.
If you cannot view the numbers normally, then there is a high chance that your girlfriend is using VoIP number to hide both the incoming and outgoing calls. Thus, you need to find a way to talk to her about it. Simply tell her what you think about the situation.