I don't know what sort of diet you are on, but I find that if I reduce the number of calories that I take in, the less likely I am to be tired after lunch. From the looks of it, 2 slices of pizza ( 2 x 370 cal ) and a small salad ( 250 cal possible underestimate) is about half of total number of calories for a 2000 calorie per day side-of-the-cereal-box diet. Also, fatty foods (such as the cheese on the pizza and the oil in the salad dressing) tend to make me tired. I try to keep the number of calories at lunch to be under 700 cal (about 1/3 of the daily total or less) and I am fine. (if still hungry after the reduced-cal lunch, then a can of sugar-free club soda & I find that I am no longer hungry.) if I eat more than this and I get tired, there is always a possibility for a small mid-afternoon cup of coffee.

When you eat, the food realaxes your body. Thats why you are tired.So take a nap.Zzzzzzzz.

you are having allergy sensitivities to what you are eating perhaps? Tuna has soy oil I presume. Pizza has yeast. Does this happen with biscuits and crackers to? Keep a food journal of what symptoms you have to what foods you eat( that are bothersome) and narrow it down.

eating more carbs, and make them clean. 100% whole wheat and not any white starchy ones. That means no unbleached and bleached flours! Try eating a seasoned grilled chicken breast with some greens and tomatoes on two toasted pieces of 100% whole wheat bread with a multivitamin after your meal. Take out the fats as they make your bloodflow slower and avoid processed flour (like in the pizza you ate) as those cause an insulin spike which will cause you to feel tired.

Your symptoms are typical of glucose intolerance.

Do you suffer from PMS or/and sweet cravings sometimes?

Tuna and cottage cheese plus salad--there are no carbs here apart from very little in veg...shortage of carbs leads to tiredness.

Consumption of too many simple sugars leads to the same symptoms.

Introduce complex carbs-wholemeal bread,pulses/lentils(cooked,not canned),buckwheat,brown rice,wholegrain pasta and always add protein to complex carbs.Green leafy veg are also of benefit in your case-contain magnesium,some plant protein and filling.

You need a balanced nutritional programme that will provide you with the right ratio of carbs/protein/fat.

I used to have the same problem in the morning when driving to work, and then the drag after eating lunch. I did research online and tried some things... the best thing that helped me was that I began making acai puree smoothies in the morning everyday. Because of acai's very high antioxidant level, it keeps you very attentive, but not with a huge drop like coffee would give you.

Eating salad could also be the problem. I've recently discovered that although you think your eating healthily, it could be the healthy food thats the cause of the problem. The body can require more energy to digest cold foods such as salad. I have this same problem and for me I found switching to eating a hot meal at lunch, normally just some soup, I was able to both loose weight and had energy. Essentially with cold foods the body needs to heat them up to break them down into the necessary proteins. Providing readily digestible hot food means the body doesn't need as much energy to extract the nutrients.

Try eating hot foods instead.