Hello,
I went back to work when baby was 10 months old. I'm a doctor in acute medicine and it is VERY busy and stressful, long shifts. I work part time, though this frequently is more than 40 hours a week anyway. I work nights, evenings, weekends, etc.
I won't lie, I was absolutely run down and exhausted for the first few months. Adjusting to life back at work after the loveliness of mat leave was very hard.
I also was ill constantly for a couple of months. I don't know if this was picking up things at work (though I thought I was immune to everything by now!!) or baby going to nursery - they catch so many things at nursery.
My boyfriend and mum frequently wondered if breastfeeding was making me more run down. I guess it's hard to know. I was also always exhausted - but I don't know if that's a toddler, my job, or bf!!
At 14 months we night weaned (I work night shifts sometimes anyway) and I have been a lot better since then, with an uninterrupted night's sleep every night. (If she wakes up in the night DP goes to her.)
We are now at 17 months, and I am 11 weeks pregnant. This is tiring, but I've not had any particularly problems with breastfeeding. In fact when I am sick and tired and struggling to be a good mother, it's quite nice that we still have that lengthy morning snuggle (she only feeds once a day now). I don't know if I want to tandem feed, just taking it a week at a time.
I only took a month to get pregnant.
My periods came back around a year (I was still night feeding at the time, but only once or twice a night). They were very irregular.
Everyone is different though. I believe you can try changing the pattern of your feeding if you are having trouble - there is a thread in conception about ttc while bf.
I am also 34. When I first went back to work, somebody told me it would take at least 6 months to feel like my old self there again - they were correct. Be nice to yourself whatever you decide, it's a tough time!