Try drinking instead of eating. Maybe you are confusing being thirsty with being hungry.
Every extra pound you gain will have to be shed afterwards so drink a glass of hot water with lemon in it.
Before you go to bed at night put a slice of lemon in a glass and pour some boiling water on it. Only just cover the lemon with the water and then it will take on the flavour. In the morning put warm water on it, to top it up and drink it.
I would recommend for breakfast a bowl of porridge, soaked in water beforehand every night, with a handful of nuts, such as hazel nuts, plus sultanas. Cook it in the morning and then mash a banana in with it. Add milk and you will have a perfect nutritious breakfast. More importantly it is very filling.
I would make sure that I have a flask of soup to take to work, so that I don't snack. Soup, such as vegetable soup is filling and makes us less hungry.
A good soup is, two onions, celery, two tins of tomatoes, garlic clove, couple of potatoes all chopped up, put into a saucepan with some water and a big handful of pasta, thrown in at the end of cooking. Pasta is very filling.
Or a cup a soup of the better variety. Anything that stops you snacking on biscuits and chocolate.
So, drink instead of eating is my advice.xx

At 7 stone 12lbs you were just shading into being underweight for you height OP. This is your body's way of telling you that you don't have enough reserves to nourish a baby.

Eat a good varied diet and you will be fine.

I'm a similar height/weight to you and was the same in the first trimester of my pregnancy. I put on a lot of weight, about 8 kg in first trimester alone, and went up over 65 kg (stopped weighing myself in the end).
At first it felt very odd to be eating so much but I decided it was a special perk of pregnancy that you can eat whatever you want . I was craving things I don't normally eat - carb heavy foods like bread and potatoes so I think it was because my body really needed the energy so try not to feel bad about it.
If you go back to eating normally again after youve had your baby the weight will go quite quickly. I was pre-pregnacy weight again by the time my son was 6 months.

I wouldn't panic either, I've a few friends who were tiny before pg and gained a lot, I gained a lot early with both my dd's and it came off just as fast. Please try not to stress, pregnancy should be a time for happiness - as long as you eat healthy stuff, don't worry. There's no harm in eating too much porridge/soup etc - if it's all cake, then you have to try and change the balance of what you're earing. Surprised dd1 didn't look like an egg I ate so many! How are you doing now?

I always gain 25 kg for every baby and lose it easily breastfeeding afterwards. Do not stress, your body feels a bit underweight for the ordeal the pregnancy is for it!