Clearblue digitals are less sensitive than most line tests. When they say 1-2 weeks pregnant they actually mean 4-5 weeks pregnant in standard medical terms. However, because doctors count the weeks from the first day of your last period, the Clearblue is saying it is 1-2 weeks since you conceived (which would be around ovulation time, which is 12-16 days on average from the first day of your period).
I am afraid that what is likely to have happened to you is that you have had a chemical pregnancy- an egg was fertilised and tried to implant, so there was some pregnancy hormone (hcg) in your system which the Clearblue picked up. However the implantation didn't succeed and the hormone is now dissipating, hence the negatives. It is very common and nothing to worry about- in the past, before sensitive home tests, women simply wouldn't know this had ever happened as it doesn't always delay the onset of the next period.
There is an outside chance of course that the pregnancy is holding on (for example the hcg might have been more dilute in the later test if you didn't use first morning urine or had been drinking a lot of water) but only time will tell I'm afraid.
I know from experience how frustrating it is to get a positive then have it fade away- happened to me at least 3 times. It can be confusing when people say that there is "no such thing as a false positive" - that is absolutely true, as you will not get a positive result without pregnancy hormone in your system, but the problem is that not all positives will lead to a viable pregnancy.
Take solace in the fact that you are young and have loads of time, and a chemical means that things are moving in the right direction- you'll get a sticky one eventually.