Didn't want to read and run but no personal experience. Given the endo then it is definitely worth getting checked over.
Where's your spouse/partner in all this? Might be time to simply tell them that you are really not feeling too well. There is little they can do to help with the feeding unless you pump but ask for help. You need to get a lot more sleep and eat better to be able to discount those two factors. You should get your iron tested in case you are anaemic. Extreme fatigue will cause nausea.
Couple of things:
Baby is 4 months old. You'll start weaning in two months and they should be getting to a weight now where they can sleep a bit longer. You could consider moving to a bottle dream feed at 11pm, leaving your partner to do it and then you cover the 5-6am feed? If you get to bed by 10pm you could get a half decent nights sleep and he/she can still be in bed by midnight having fed the baby.
Can they bring you breakfast in bed in the morning before they go to work? Something nutritious like porridge or muesli and greek yoghurt. You feed the baby, eat your breakfast and then have that weird short nap that babies do for about 30-60 mins first thing and then you both get up for the day. You can then focus on getting out of the house. If you've had a half decent nights sleep, you can shower and dress while the baby has a short nap, then move to a quick feed and out and in a couple of months use the time to do a quick spot of baby porridge.
Can you meal plan with your partner so he/she cooks something every night that you can have as a quick lunch of leftovers? Can you hand over the baby for a stretch at the weekend so you can have some time to yourself and take a lengthy nap?
Beware falling into the trap where he/she will take the baby nowhere without you. Once fed, they should be able to take the baby out in the pram for a bit and give you a break. Equally the trap where you are on maternity leave and are therefore de facto responsible for all the household chores, the baby and expected to accompany all outings and ensure he/she gets a "day off". You've had a child, shit changes.
Look after yourself.