If you have never had any intercourse since you had your last period, then yes, you cannot be pregnant. If you did have intercourse during this period but without any ejaculation, you do have chances of conception as the pre-ejaculate
contains enough sperm to cause conception during the fertile or ovulation period. Your symptoms may be seen in early pregnancy
, but you need evidence as the pregnancy test which was done. If you are still doubtful, you can go for a blood pregnancy test which is very specific and sensitive and also get a trans-vaginal sonogram, which is conclusive. Hope this helps.