Low carb = low fiber = problem!

First of all you don't "go" on a lifestyle change! You only need to change your lifestyle once and continue on the rest of your days. One only goes "on" and eventually "off" a "diet".
That being said, I eat a slice of whole grain bread a day or Smartbran cereal in the morning. Other than that, I eat tons of veggies, some fruit, protein such as peanut butter, eggs, chicken, fish, beans and steak about 1-2x a month. Sometimes I have oatmeal w/flax, chia seeds, nuts and fruit. I measure out each carefully. I drink 4 oz of prune juice each morning and 2 huge glasses of water about 1/2 hour before breakfast. At night I take a 400 mg capsule of magnesium citrate. That is very calming and great for constipation.
I eat this way and eat back my exercise calories and still lose. I also have chocolate every day;-))

So, there are different kinds of fibre, right? Fibre you can digest (soluble fibre) and fibre you can't digest (insoluble fibre). Soluble fibre slows things down and will help with diarrhea. Insoluble fibre speeds things up and will help with constipation. Soluble fibre is great for making you feel full, helping your body absorb nutrients (remember, it's slowing things down), and is found in things like oatmeal, flax, apples, nuts, etc. Insoluble fibre is what most people think of as fibre -- roughage. Both are healthy and good for you and you need a balance of them. There's also fermentable fibre, which is fibre that ferments while it's in your colon. Gross sounding, I know. Soluble fibre generally ferments, but especially the fibre in berries, apples, and oats. This is the fibre that's considered especially good for reducing your risk of colon cancer. But in general, it's all good.

What you may want to do is start focusing your carb allotment on making sure you get a good balance of different fibre. If all of your fibre right now is going towards soluble fibre, it's not surprising that you end up constipated. I don't know how many carbs you're trying to stay under, but it may be helpful to you to start counting 'net' carbs, so that you're more inclined to eat fibre instead of just low carb. To get net carbs, you basically subtract the fibre in a given food from the total number of carbs. Assuming you're in the US. I think the UK lists fibre and carbs separately on their food labels, so the total carbs on a UK label is the US version of 'net carbs' I think . I wouldn't bet my life on any of that.

If you don't want to mess with calculating net carbs, or someone much wiser than me comes along to explain why net carbs are bull, then I would suggest you focus on insoluble fibre to help with the constipation and drinking more water. You always want to drink lots of water, but especially when you're constipated.