Hi welcome to the health care magic 
I will solve your query and will give my opinion in this answer..... 
Let me explain you false positive urine card test causes...... 
_
abortion history in last 6 week 
_Test not done as per kit insert 
_reading not taken in prescribed time in kit insert 
_expired kit 
_urine contamination with blood 
_trophoblastic tumour etc... 
I advise you to repeat urine card(at home)  or blood 
hcg estimation done (at hospital) 
I need to know USG done by Transvaginal route or transabdominal route?? 
Tvs 
sonography able to visualize 
gestational sac 5 week after LMP if conception occured 
And for Transabdominal time period is 6 week after LMP 
Urine card positive 5 week after LMP 
And remember USG confirmation needed to rule out 
pregnancy Hope I have solved your query 
Take care