Hi Anna,
I just wrote this to another girl that was havingtroubles with the same thing. So I thought that I would post it for you also.
Malakai is a textbook baby and this applies to everything. His routine runs to the seconds not minutes. So if I miss cues or jump on early cues then we have a battle on our hands. Thats great for us because I can predict what he needs most of the time. EG. There are 3 stages I find to his getting tired phase. Stage one He starts with short bursts like a scream but not If I put him down for a nap now I have no hope he is not really ready he is just saying that he wants less to be happening so he can wind down. If he does sleep it is usually approx 30 min Stage 2 he rubs his eyes. If I put him down for a nap/sleep now he will put himself to sleep and sleep until I wake him up. Stage 3. He is yawning. If he makes it to this stage he is overtired and again approx 30 min is all I will get from him. The time between the stages varies depending on what how active his A time was. It can be 5 min from stage 1 to 3 but it can also be like 20 min. As for the OT UT times - I'm not sure about that because Malakai will sleep approx 30 min under both situations. I guess you just learn your child and how they respond under different circumstances.
We have followed EASY since about 2 months old so for 2 months now we have been on 3, 3.5 and now 4 hour EASY. Growth spurts do effect but I find at night more so. Naps are not usually affected with us.
Maybe the teething is effecting her. Did you say that she was teething also. Apparently babies temperatures rise when the teething may be effecting them, so that might be something that you could try. I would be saying cues and teething are the most likely. Are you sure that your LO is getting enough to eat during the day this can also effect sleep at night. I find this is the biggest thing when bubs is waking at night. As your LO is 6 months this is the next growth spurt and he may need more to eat to help him grow.