Have 14 fish then i got 7 more then all of a sudden they started dying 7 of my fish died after i got the 7.?

Ok, heres the problem, i had 14 fish from petco (4 bleeding heart tetras, some orange tetras, 4 neon fish, and 3 guppies) then i got 7 more from inderground fish (in cloqet. i got 3 Angelfish,and 4 swordtails) then after I put the 7 more in the tank (i had 21 fish, big tank they fit fine!) a few days later ( 3 or 4) my female red swordtail (named caroline) gave birth in the night (we did not get to see the babies when we woke up and she was not pregnate, the babies must of died) two days after birth my mom foung her and a nother male swordtail stuck in a bush, that night they died (we got rid of their bodies) then two days later my fish are fine then i go upstairs for about 15 mins, then i come back down and my red guppie and a nother swordtil fish are on the ground dead! but when the other fish died we noticed that,that day,they were fine then they get weak then they die! it happend to 4 others! the 7th fish died this morning but i knew it would (i saw how weak it was) whats happening! right now i have 14 fish again and they seem fine. i think that they guy ho sold up the 2nd bunch of fish gave us sick ones. but how do they just die like that?!