It's such a guessing game, isn't it? If the GP didn't find anything, then it could be teeth emerging (??) Our DS2 screamed for weeks at nighttime,with EVERY tooth that he cut. Oh my. Panadol and Nurofen together couldn't calm the crying and we'd be up for hours holding him. The tooth or teeth would emerge and we'd have a respite of a few weeks and then back into it again.
Or separation anxiety - I think that happens around 6 months.
Not sure what age night terrors start as DS1 experienced those, but I think that's more toddler age and would happen after the first hour or two of sleep. Watching The Night Garden on DVD seemed to stop the screaming. It's like a drug for kids - as soon as they hear the music they go quiet, but again, probaby more a toddler thing.
Can you try a warm bath to calm them down (probably not what you're wanting to do in the middle of the night).
We found when the teething screaming wouldn't stop after hours that a 5-10 min drive was the only thing that worked. Not fun when we live in the country where it's pitch black some nights and the nearest road is 100km/hr with kangaroos around. Talk about desperate measures!

Hope you get some respite soon. A few weeks of screaming can feel like an eternity and you really start to dread the evenings. Hang in there!