It all depends on how long your messages were if they are long ones you may have been charged for 2,4 or maybe 6 messages keep your messages as short as possible by using abbreviations or text speak.
If you told us what supplier you were with (O2, Tesco, Vodafone etc) and which contract you were on (Pay as you go I assume but which one) then we may be able to help you.
I'm on tesco pay as you go, but this £5 top up was a good will favour because of them making a cock up over me cancelling a monthly contract.
Hard to explain, but my text should never cost that much surely.
I had £5 top up voucher y/day,. made 3--4 texts and it just said I have £2 something left on mobile.
Just a thought. Do you have the internet turned on? If you were previously on contract there would probably have been an allowance for this. PAYG probably will not, so you will be paying for any updates etc that are coming through.
That could be the answer. I don't think you will have a data allowance on PAYG. I don't on mine I have to pay, but it's not Tesco. Turn the internet off, only use Wifi and see if your balance still goes down quickly.
After you've turned it off why not check your balance send a text and check again. That way you will know exactly how much a text is costing you.
The easiest way to check is to try and access something on the web now, AB or BBC news for example. You should get 'no network connection' if it's all turned off.