Am I doing things wrong?

Firstly you should be eating at least 1200 cals a day and eating too little doesn't make you put on weight or stop weight loss.
Secondly have you been weighing and logging your food? As you will put weight on if you are eating more calories than you burn.

If you maintain at 2600 cals then eating at 2100 would give you a 1lb per week loss and 1600, approximately 2lb per week. At 23 stone you are OK to aim for 2lb per week loss.

These figures are based upon your exercise/daily movement at maintenance (2600 cals) and so, if you do extra i would suggest eating back about half of those gained calories.

My advice is:

• Target 1600 cals per day.
• Weigh and record all your food.
• Be consistent.
• If you slip up and eat too much, don't beat yourself up about it but get back on plan ASAP.
• Don't be in a hurry to lose the weight - it takes time and the people who do it slow and steady seem to have the most success.
• Don't eliminate whole food groups or foods you love in favour of "healthy" foods. Eat a balanced diet which includes the stuff you like (within your calorific target)
• If you enjoy exercising (such as walking) great - go for it there are solid health benefits, but what is required for weight loss is calorific deficit and this can be achieved without exercise.

If you were truly eating as little as 1100 (btw 1200 is minimum for women,to get proper amounts of nutrition) you should be losing weight.
Have you been measuring and weighing everything you eat with a scale? If not start doing that and definately look at the amount you are consuming 1100 is a tiny amount and as someone who is a lot smaller and older than you I can't survive on that little.
I don't really think you have been eating that little but , when you start tracking don't make your deficit so aggressive , slow and steady wins the race.
Good luck x

Just saw you don't have a food scale
Definately invest. It's the best tool to lose weight.