How long have you been low-carbing? Some people get what is called "Atkins flu" which includes headaches. It doesn't last long; persevere, and drink plenty of water - at least 2 litres a day. I fill a 2-litre jug every morning and drink some every time I pass; it's usually empty by bed-time.
If you're eating very low carb (around 25g daily) you may not have a bowel motion every day; you're not eating enough 'waste' to create a stool every day. You can eat rhubarb sweetened with (eg) Splenda powder, with either cream or live yoghurt. Live plain yoghurt is good for your innards anyway. Flax seed is good, or plain bran (wheat or oat) added (eg) to casseroles. When all else fails I use Andrew's Liver Salts, but that's only about once a year. It's not a good idea to use laxatives too frequently!
You could search "Constipation" through the search facility at the top of the index page. There's been lots of discussion about it on here previously.
Hope this helps
Viv 8)