car seat screaming

[SammyRo]

My normally calm baby, who is now 7 weeks, seems to have developed an issue with the car seat. The last 3 days I've been up at my parents during the day. On the way there in the morning, Alyssa is fine in the car, in fact yesterday morning, she was chatting away. However, the nightmare comes when I click her car seat into the car base and start to drive home. She used to grizzle at first but then was quite happy and often went to sleep as we drove. On Monday, she started screaming, and I mean inconsoble crying type screaming, so I pulled over, got in the back seat and stroked her hands and stomach til she calmed down, which she did until we were round the corner from home. I thought that it was because we had put her in and out of her car seat a few times as we had issues with the straps and so this could have unsettled her. Anyway, Tues night, I left earlier, thinking that maybe she was also ready for a feed on monday, and so by leaving earlier would get home before she was hungry - nope, not a chance, managed to get further before the screaming started, however, she did not stop until i got home and pulled her car seat out of the car. So last night, i decided to feed her just before i put her in her car seat, and she was a very happy and content baby until again, she started grizzling as i clicked the seat in and then screaming by the bottom of the hill. I pulled, tried to calm her in her seat, which didn't work this time, decided to change her nappy and knew she was not hungry. she calmed down until she went back into the capsule. I phoned hubby and he suggested checking that there was nothing in the seat that was causing the issue, so i pulled over again, rocked her and she settled within seconds, checked the seat and couldn't feel anything, and so put her back in, again as soon as she was in, she was screaming, and nothing else short of taking her out again would stop her until 5 mins from home when i think she just exhausted herself.
Anyone got any similar experience or suggestions? I thought at one point about abadoning the car and walking the last 10k home! I really don't know what is causing this, as i said in the morning she is fine, and we've not had this issue at night before. Its the most awful thing though, having her scream and not knowing what to do about it.

Thanks in advance
Sam

eta: its a safe in sound capsule, the type you hire from the red cross, and thinking about it, mon morning, she pooed everywhere so we had to pull the lining out and wash it, so this could be related, although if it was, it would be causing issues the whole time and not just at night.