Could i be pregnant while being on depo injection? please help?

i have had 3 lots of depo injections now and my next one isn't due till end of may, i have got 2 kids already this is why i need help i have bin reading on the internet that u can get pregnant while on depo, i have bin really tired lately and feelin sick in the mornings sometimes, i feel different but i have bin spotting little bits every day and i have done a cheap tescos pregnancy test and it was negative but i just worried that i am pregnant and that there might b somethin wrong wiv the baby and that is why im spotting. can someone help please cause when i went for my last injection she said the spotting should stop now cause i am on my third injection.