I had my periods on 26th of march and the next were due on 26th April but they didn't came, I waited for few days then I met up with the doctor and she said it do happens and she advised to take pills for period if they don't come till 26th of May. On 26th of may before take the pill I took pregnancy test and it was positive, I had done by pregnancy test on 18th may and that time it was negative.
Is this OK?
I have heard that there are problems if you get pregnant unusually, is that true?

I had my periods on 26th of march and the next were due on 26th April but they didn't came, I waited for few days then I met up with the doctor and she said it do happens and she advised to take pills for period if they don't come till 26th of May. On 26th of may before take the pill I took pregnancy test and it was positive, I had done by pregnancy test on 18th may and that time it was negative.
Is this OK?
I have heard that there are problems if you get pregnant unusually, is that true?

What is unusual ? You tested way to early first time, most pregnancy tests not work till you are one or two week, past date for cycle.

Also the time of day, you take test and effect its validity. Early morning, first urine of the day is always best.

Sorry, but there is nothing unsual about this, Except that doctor did not do blood test to confirm not pregnant before giving you medication

False positives are rare. Congratulations.

I had visited doctor on 4th may and she had asked me to do all the tests, I did all tests on 8th of May and everything was normal.Till 18th May my pregnancy test was negative and when I had a test on 26th May it was positive.It means that I got pregnant after 50days of periods, is this OK?

IT is okay. There are women who have irregular periods, often several months between periods, who get pregnant after not having a period for a few months. Becoming pregnant takes several factors to line up perfectly, ripe lining, ovulation, insemination, and implantation, and the convergence of these factors can happen any time in your cycle, even it is a LONG cycle.

So it is perfectly fine and normal. Congratulations on your coming child.