Electronic – PORTB Interrupt On Change changes state unintentionally

I am running below code in a PIC16F887 that runs at 20 MHz. I feed PORTB.0 a signal of 1 KHz from my signal generator. However, whenever the signal is low, PORTC.1 toggles at a rate of either ~5usec or ~10usec. Also, I've connected an OP-AMP's output that gives 0V-4V, still the same.

I have tried enabling weak pull ups inside, didn't work.

The microcontroller is on an EasyPIC5 development board, so I can enable and disable either pull-up or pull-down resistors that are 10K. Didn't work, either.

Is this some kind of silicon problem(?) or is there a mistake in my code?

Here is a screenshot from the oscilloscope that probes PORTC.1:

Here is my code, please omit off-topic part of it:

volatile unsigned int fiftymicros=0;
volatile unsigned int milis=0;
volatile unsigned char fiftymicroscounter=0;
volatile unsigned int data_time=0;
volatile unsigned int milis_ctr=0;
unsigned long int data_bar[20]={0};
void interrupt()
{
     if(INTCON.T0IF)
     {
      TMR0=198;
      INTCON.T0IF=0;
      fiftymicros++;
      if(++fiftymicroscounter==40) { fiftymicroscounter=0; milis++; }
     }
     if(INTCON.RBIF)
     {
      PORTC.F1=~PORTC.F1;
      INTCON.RBIF=0;
     }
}
void main()
{
     ANSEL =0;
     ANSELH=0;
     TRISA=0; PORTA=0;
     TRISB=0x01; PORTB=0;
     TRISC=0; PORTC=0;
     TRISD=0; PORTD=0;
     TRISE=0; PORTE=0;
     OPTION_REG.INTEDG=1;
     OPTION_REG.T0CS=0;
     OPTION_REG.PSA=0;
     OPTION_REG.PS0=1;
     OPTION_REG.PS1=0;
     OPTION_REG.PS2=0;
     //OPTION_REG.B7=0;
     //WPUB=0x80;
     INTCON.TMR0IE=0;
     INTCON.RBIE=1;
     IOCB.B0=1;
     INTCON.GIE=1;
     UART1_Init(9600);
     Delay_ms(100);
     milis_ctr=milis;
     while(1)
     {
     }
}