Missed period. What should i do next?

Hello doctors, I have missed my period this month, I was expecting them on 1st August as last lmp was 1st July, I usually get them on time... but this month it is delayed by 10 days... I have checked on Home pregnancy test kit but it showed negative... Good chance that I am not pregnant as we are already taking precautions but just 5%chance is there if by chance the contraception failed. That is also not likely. I am also breastfeeding my 8 month old baby... 1.Till this time period were regular, but is it possible to miss it after 8 months of breastfeeding? 2.Or can the result be false negative on the test kit? 3. What should I do next? Should I wait or should I go for an ultrasound..? Pls suggest. Thank you in advance.