Am I eating enough?

I have no idea about whether what I'm eating is enough and how many calories it is roughly? 

I saw programme recently where they worked out the woman should have been eating over 2,000 calories a day cos of her current weight and height..  how are you meant to lose?

I'm currently only eating:

Breakfast 1 small bowl of special K with semi skimmed milk or oats instead

lunch:  one sandwich with 2 slices of weightwatchers wholemeal bread (lettuce tomato and either ham or cheese) ,  or a bowl of salad (lettuce,  raw onion,  tomato and fish. ie tinned mackerel or tuna) or soup

dinner:  half a plate of veg with small portion of fish or lean white meat and small portion of carbs

only exception is sunday dinner when I add a small yorkshire pudding (auntie bessies mini's and may have lamb chop instead of chicken)

one piece of fruit during the day and sometimes (but not everyday)  1 yoghurt. 

Is this 1200 calories?.. I don't know if its too much and I'm not losing cos eating too much or not enough and I'm in starvation mode?  or maybe I should be eating much more calories for 5ft 3 and half and 12 and half stone?   

Kati