I can only think your body is releasing hormones due to the stress of getting up in the night. Ive found that before when I've been up late or had to work at nights. I haven't found it with breast feeds or pumps, but it seems the logical explanation.
You're on a pump? I'd be tempted to get advice from a DSN who's familiar with post-natal issues (maybe a pregnancy one). How high do your sugars go? If you're confident you could try a tiny corrective dose on your pump, but I'd start off very, very cautiously and see the effect.
My other thought is could you be going low earlier in the night and then having a compensatory high later?