I eat a lot of veggies and fruit, and I eat legumes and I do eat some grains, and some oil or occasionally nuts/seeds. My calorie intake is probably a bit lower than it should be as I am underweight and I am trying to lose weight (not a lot, I like to be 47kg-50kg and I was 50.4kg when last weighed couple days ago). I could get more calories by eating more food without changing the structure of my diet, I am just restricting at the moment because, well because I am a bit too strict on myself to be honest and It'd probably be healthier if I could losen up a bit more about my weight. However I am only mildly underweight there are people this thin naturally who are healthy so I figure as long as I get all my nutrients (and I do take a potent multi-vitamin just in case I could do with an added boost, it's easier to assimilate nutrients from food than pills I know, I am not relying on it, it's just a safeguard) I can be healthy and maintain my weight at a range I can live with, oppose to gain weight closer to "normal" then freak out and get stuck in the cycle of starving and binging that results, and also not feel the need to continue losing weight beyond my set low point, 10 years on from AN, though I bet it still sounds a bit not quite right, I am healthier than I've ever been.
6 months is a pretty good stint I would have expected you to be feeling good after six months on a vegan diet. Were you just eating too many gassey type foods when you were vegan perhaps?, too many beans broccoli cabbage etc (all these foods are healthy but too much can make some people gassey and bloated), Also were you eating a lot of white flour, sugar, breads? Stodge can bloat people up. Maybe probiotics could have helped?
A vegan diet is high in fibre, usually this helps with elimination but if you weren't drinking enough water to flush through the fibre it can't do it's job and can cause constipation, bloating and abdominal discomfort.
The feeling tired a lot isn't a good sign, maybe this was caused by the digestive problems or maybe you were missing something from your diet. Were you taking a B12 supplement? Did you eat a variety of veggies at least 4 serv including dark green leafy and yellow/orange? At least 3 servs fruits, what about legumes, were you eating 2 servs? did you only eat beans or was there a variety of beans, peas, lentils, tofu, hummas, etc.? Did you get essential fats? linseed for the essential omega-3? sunflower oil or seeds for essential omega-6? Were you getting enough calories in general? Were you getting too many calories/eating a lot of junky vegan foods?
Some vegans can develop dietary deficiencies (as can omnivores), it's generally the ones who are vegan exclusively for ethical reasons that don't really pay enough attention to what makes up a nutritionally balanced vegan diet and basically just end up eating a lot of pasta, bread, potato wedges, fruit juice, fake meat products, immitation cheese products, vegan cakes and sweets, basically whatever they think would be tasty, provided it's vegan, becomes their meals. But in some cases vegans who do try to eat balanced and healthfully and take a B12 can have low iodine or low vit D levels. People may not think to get enough sun exposure, they may wear a lot of high SPF everyday, live in a region that has low sun intensity. Now days most soymilk ricemilk soy yoghurts are all fortified with vitamin D for this reason, but someone who is trying to eat whole unprocessed foods could still miss out. And iodine seems to be low for most people that's why they add it to salt, a lot of people are cutting down on salt however you can also eat kelp like nori seaweed or spirulina for iodine.
If you were getting all the nutrition above, and you still felt unhealthy like you say, considering what you've said about how well you're doing on a paleo diet, I would guess you have a wheat/gluten allergy or intolerance. You'd need to see a doctor to diagnose that obviously, but if you are happy with your current diet and had a bad experience with previous attempt at veganism, I understand you probably want to continue to cut out all grains and legumes, and even though I think meat is unhealthy in a lot of ways (too much fat, too much cholesterol, too much protein, etc.) cutting it out if you weren't going to add legumes back in at least I fear would leave you lacking in necessary nutrients, can not possibly recommend!
So, keep up what you're doing I guess? Well done on losing weight and gaining health!